Schedule I Condensed Financial Information of Registrant Parent Company Only - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	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, 2019	Dec. 31, 2018
Expenses
General and administrative									$ 9,939	$ 24,569	$ 24,429
Operating income	$ 226,758	$ 208,380	$ 205,495	$ 201,866	$ 195,682	$ 184,100	$ 189,448	$ 188,724	144,196	842,499	757,954
Interest income									282	20,014	11,307
Income before income taxes									42,636	555,986	533,558
Income taxes									(1,901)	1,705	1,441
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
Unrealized loss on cash flow hedges									0	(42,954)	(22,124)
Comprehensive income attributable to common stockholders									42,662	503,010	501,495
Parent Company
Expenses
General and administrative									0	0	78
Total operating expenses									0	0	78
Income (Loss) from Equity Method Investments									0	532,699	516,116
Interest income									282	13,265	7,581
Income before income taxes									282	545,964	523,619
Income taxes									0	0	0
Net income attributable to common stockholders									282	545,964	523,619
Unrealized loss on cash flow hedges									0	(42,954)	(22,124)
Comprehensive income attributable to common stockholders									$ 282	$ 503,010	$ 501,495