Line of Credit Payable (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Revolving credit facility borrowing capacity	$ 650,000,000
Revolving credit facility weighted average outstanding balance	25,489,000
Revolving credit facility weighted average interest rate	1.10%
Maturity date	Jan. 01, 2019
Option to extend maturity date	Jan. 01, 2020
Option to increase facility size	1,000,000,000
Line of credit payable	41,900,000	0
Line of credit facility available for future borrowings	$ 608,100,000
Line of Credit | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.925%