Real Estate Investments - Revenue and Earnings of Acquisitions (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Real Estate [Line Items]
Real estate rental revenue	$ 76,952	$ 79,770	$ 79,405	$ 77,137	$ 79,102	$ 78,243	$ 74,226	$ 74,856	$ 313,264	$ 306,427	$ 288,637
Net income	$ 5,426	$ 79,662	$ 31,821	$ 2,379	$ 62,095	$ 580	$ (2,886)	$ 29,398	119,288	89,187	111,601
Property Acquired [Member]
Real Estate [Line Items]
Real estate rental revenue									13,112	6,797	16,260
Net income									$ (1,688)	$ (2,748)	$ (3,168)