Quarterly Operating Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Operating Results (Unaudited)
Revenues	$ 272,952	$ 281,639	$ 302,896	$ 257,680	$ 280,852	$ 289,308	$ 317,447	$ 271,446	$ 1,115,167	$ 1,159,053	$ 1,193,638
Total operating expenses	261,677	239,346	243,297	233,474	235,346	241,220	255,592	245,005	977,794	977,163	1,052,633
Operating income	11,275	42,293	59,599	24,206	45,506	48,088	61,855	26,441
Net income	45,414	33,545	45,918	17,916	77,756	91,586	51,262	38,455	142,793	259,059	153,004
Income attributable to common stockholders	$ 41,208	$ 27,829	$ 40,756	$ 13,110	$ 73,123	$ 86,002	$ 45,681	$ 32,809	$ 122,903	$ 237,615	$ 132,546
Income attributable to common stockholders per share - basic and diluted (in dollars per share)	$ 0.18	$ 0.12	$ 0.18	$ 0.06	$ 0.32	$ 0.38	$ 0.20	$ 0.15	$ 0.54	$ 1.05	$ 0.59