| 
           Balance at December 31,  | 
        ||||||||
| 
           Description  | 
        
           2013  | 
        
           2012  | 
        ||||||
| 
           Unsecured Revolving Credit Facility  | 
        ||||||||
| 
           Matures February 20171; maximum borrowing level of $200.0 million and $163.5 million available at December 31, 2013 and 2012, respectively; interest at LIBOR + 1.95%2 or 2.12% at December 31, 2013 and interest at LIBOR + 2.40%2 or 2.61% at December 31, 2012  | 
        $ | 145,000,000 | $ | 94,624,200 | ||||
| 
           Unsecured Term Loan  | 
        ||||||||
| 
           Matures August 20183; interest at LIBOR + 1.80%2 or 1.97% at December 31, 2013 and interest at LIBOR + 2.60%2 or 2.81% at December 31, 2012  | 
        230,000,000 | 125,000,000 | ||||||
| 
           Notes Payable Secured by Properties under Construction—Variable Rate  | 
        ||||||||
| 
           Generally interest only; maturing at various dates through 2016; interest at LIBOR+2.00%-2.50%, ranging from 2.17% to 2.67% at December 31, 2013 and interest at LIBOR+2.00%-2.50%, ranging from 2.21% to 2.71% at December 31, 2012  | 
        144,388,705 | 72,156,149 | ||||||
| 
           Mortgage Notes Payable—Fixed Rate  | 
        ||||||||
| 
           Generally due in monthly installments of principal and interest; maturing at various dates through 2022; interest rates ranging from 5.42% to 6.78% at December 31, 2013 and interest rates ranging from 5.42% to 6.78% at December 31, 2012  | 
        276,504,111 | 338,765,294 | ||||||
| 
           Mortgage Notes Payable—Variable Rate  | 
        ||||||||
| 
           Due in monthly installments of principal and interest; maturing at various dates through 2020; interest at LIBOR + 1.25%-2.94%, ranging from 1.42% to 3.11% at December 31, 2013 and interest at LIBOR + 1.25%-3.25%, ranging from 1.46% to 3.46% at December 31, 2012  | 
        61,186,570 | 69,171,405 | ||||||
| 
           Net premium on acquired indebtedness  | 
        64,688 | 191,720 | ||||||
| 
           Total mortgage and other indebtedness  | 
        $ | 857,144,074 | $ | 699,908,768 | ||||