Invesco Ltd. | 2013 | FY | 3

The following table shows significant unobservable inputs used in the fair value measurement of level 3 assets and liabilities:
Assets
 
Fair Value at December 31, 2013 ($ in millions)
 
Valuation Technique
 
Unobservable Inputs
 
Range
Guaranteed investment contracts
 
15.2

 
Discounted cash flow
 
Discount rate
 
4.4%
 
 
 
 
 
 
Mortality assumption
 
Standard UK mortality tables with a long-term rate of improvement of 1.25%
The components of net periodic benefit cost in respect of these defined benefit plans are as follows:
 
Retirement Plans
 
Medical Plan
$ in millions
2013
 
2012
 
2011
 
2013
 
2012
 
2011
Service cost
(4.4
)
 
(4.5
)
 
(4.4
)
 
(0.2
)
 
(0.3
)
 
(0.5
)
Interest cost
(19.0
)
 
(19.6
)
 
(19.1
)
 
(1.9
)
 
(2.1
)
 
(2.3
)
Expected return on plan assets
18.5

 
17.4

 
17.6

 
0.6

 
0.5

 
0.5

Amortization of prior service cost/(credit)
(0.1
)
 
—

 
—

 
2.0

 
2.0

 
2.0

Amortization of net actuarial gain/(loss)
(2.0
)
 
(2.2
)
 
(1.2
)
 
(0.3
)
 
(0.2
)
 
(0.3
)
Net periodic benefit cost
(7.0
)
 
(8.9
)
 
(7.1
)
 
0.2

 
(0.1
)
 
(0.6
)

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