Quarterly Results of Operations (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands										3 Months Ended										12 Months Ended
	Dec. 12, 2019	Sep. 12, 2019	Jun. 13, 2019	Mar. 14, 2019	Dec. 13, 2018	Sep. 17, 2018	Jun. 14, 2018	Mar. 31, 2018	Mar. 15, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues										$ 237,537	$ 222,513	$ 220,746	$ 214,002	$ 226,039	$ 232,687	$ 220,975	$ 218,276		$ 187,609	$ 894,798	$ 897,977
Operating income										226,758	208,380	205,495	201,866	195,682	184,100	189,448	188,724		144,196	842,499	757,954
Net income										100,713	146,515	154,127	152,926	144,631	132,024	141,359	114,103	$ 44,537	44,537	554,281	532,117
Net income attributable to common stockholders										$ 98,631	$ 144,435	$ 152,049	$ 150,849	$ 142,541	$ 129,912	$ 139,044	$ 112,122		$ 42,662	$ 545,964	$ 523,619
Net income per common share
Basic and diluted (in dollars per share)										$ 0.21	$ 0.31	$ 0.37	$ 0.37	$ 0.37	$ 0.35	$ 0.38	$ 0.33
Dividends per share (in dollars per share)	$ 0.2975	$ 0.2975	$ 0.2875	$ 0.2875	$ 0.2875	$ 0.2875	$ 0.2625	$ 0.2625	$ 0.16	$ 0.2975	$ 0.2975	$ 0.2875	$ 0.2875	$ 0.2875	$ 0.2875	$ 0.2625	$ 0.1600		$ 0	$ 1.17	$ 0.9975