9. Identified Intangible Assets and Liabilities
The following summarizes our identified intangible assets (primarily acquired above-market leases) and liabilities (primarily acquired below-market leases) as of December 31, 2013 and 2012.
| Balance as of December 31, | |||||||
| (Amounts in thousands) | 2013 | 2012 | |||||
| Identified intangible assets: | |||||||
| Gross amount | $ | 605,915 | $ | 767,365 | |||
| Accumulated amortization | (282,593) | (352,035) | |||||
| Net | $ | 323,322 | $ | 415,330 | |||
| Identified intangible liabilities (included in deferred revenue): | |||||||
| Gross amount | $ | 892,487 | $ | 902,525 | |||
| Accumulated amortization | (382,002) | (341,536) | |||||
| Net | $ | 510,485 | $ | 560,989 | |||
Amortization of acquired below-market leases, net of acquired above-market leases resulted in an increase to rental income of $52,861,000, $54,215,000 and $61,869,000 for the years ended December 31, 2013, 2012 and 2011, respectively. Estimated annual amortization of acquired below-market leases, net of acquired above-market leases for each of the five succeeding years commencing January 1, 2014 is as follows:
| (Amounts in thousands) | ||||
| 2014 | $ | 45,588 | ||
| 2015 | 42,095 | |||
| 2016 | 40,489 | |||
| 2017 | 35,173 | |||
| 2018 | 33,408 |
Amortization of all other identified intangible assets (a component of depreciation and amortization expense) was $64,330,000, $49,597,000 and $52,632,000 for the years ended December 31, 2013, 2012 and 2011, respectively. Estimated annual amortization of all other identified intangible assets including acquired in-place leases, customer relationships, and third party contracts for each of the five succeeding years commencing January 1, 2014 is as follows:
| (Amounts in thousands) | ||||
| 2014 | $ | 29,238 | ||
| 2015 | 23,869 | |||
| 2016 | 20,689 | |||
| 2017 | 17,260 | |||
| 2018 | 12,860 |
We are a tenant under ground leases at certain properties. Amortization of these acquired below-market leases, net of above-market leases resulted in an increase to rent expense of $4,357,000, $1,328,000 and $993,000 for the years ended December 31, 2013, 2012 and 2011, respectively. Estimated annual amortization of these below-market leases, net of above-market leases for each of the five succeeding years commencing January 1, 2014 is as follows:
| (Amounts in thousands) | ||||
| 2014 | $ | 3,430 | ||
| 2015 | 3,430 | |||
| 2016 | 3,430 | |||
| 2017 | 3,430 | |||
| 2018 | 3,430 |