Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 86,541	$ 85,139	$ 86,700	$ 65,715	$ 168,443	$ 66,635	$ 63,107	$ 52,070	$ 324,094	$ 350,255	$ 219,414
Income attributable to unvested participating securities, basic EPS									(1,893)	(2,526)	(1,835)
Net income used in calculating basic EPS									$ 322,201	$ 347,729	$ 217,579
Weighted-average vested shares									43,854	45,598	47,411
Basic EPS	$ 1.99	$ 1.95	$ 1.96	$ 1.46	$ 3.73	$ 1.46	$ 1.36	$ 1.12	$ 7.35	$ 7.63	$ 4.59
Income attributable to unvested participating securities, diluted EPS									$ (1,888)	$ (2,521)	$ (1,833)
Net income used in calculating diluted EPS									$ 322,206	$ 347,734	$ 217,581
Dilutive effect of incremental shares to be issued for contingently-issuable shares									131	91	42
Weighted-average shares used in calculating diluted EPS									43,985	45,689	47,453
Diluted EPS	$ 1.99	$ 1.94	$ 1.95	$ 1.46	$ 3.72	$ 1.46	$ 1.36	$ 1.11	$ 7.33	$ 7.61	$ 4.59