Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 63,107	$ 59,326	$ 115,176	$ 104,502
Income attributable to unvested participating securities, basic EPS	(430)	(491)	(859)	(861)
Net income used in calculating basic EPS	$ 62,677	$ 58,835	$ 114,317	$ 103,641
Weighted-average vested shares	45,984	47,658	46,142	48,104
Basic EPS	$ 1.36	$ 1.23	$ 2.48	$ 2.15
Income attributable to unvested participating securities, diluted EPS	$ (430)	$ (490)	$ (857)	$ (860)
Net income used in calculating diluted EPS	$ 62,677	$ 58,836	$ 114,319	$ 103,642
Dilutive effect of incremental shares to be issued for contingently issuable shares	87	50	80	37
Weighted-average shares used in calculating diluted EPS	46,071	47,708	46,222	48,141
Diluted EPS	$ 1.36	$ 1.23	$ 2.47	$ 2.15