Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 535,142	$ 577,588	$ 570,603	$ 488,602	$ 479,278	$ 543,516	$ 550,299	$ 474,118	$ 2,171,935	$ 2,047,211	$ 1,921,904
Net income	31,545	85,728	60,699	37,171	63,186	51,812	56,061	52,051	215,143	223,110	166,418
Net income available for common shareholders	$ 31,545	$ 85,728	$ 60,699	$ 25,843	$ 58,020	$ 46,646	$ 50,895	$ 46,885	$ 203,815	$ 202,446	$ 145,754
Net income available for common shareholders per common share: Basic and diluted (in dollars per share)	$ 0.19	$ 0.52	$ 0.37	$ 0.16	$ 0.35	$ 0.30	$ 0.34	$ 0.31	$ 1.24	$ 1.30	$ 0.97
Distributions per common share (in dollars per share)	$ 0.52	$ 0.52	$ 0.52	$ 0.51	$ 0.51	$ 0.51	$ 0.51	$ 0.50