Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 45,176	$ 43,401
Net income used in calculating basic EPS	$ 44,806	$ 42,957
Weighted-average vested shares	48,550	52,643
Basic EPS	$ 0.92	$ 0.82
Net income used in calculating diluted EPS	$ 44,806	$ 42,957
Dilutive effect of incremental shares to be issued for contingently issuable shares	24	48
Weighted-average shares used in calculating diluted EPS	48,574	52,691
Diluted EPS	$ 0.92	$ 0.82
Stock Compensation Plan [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income attributable to unvested participating securities, basic EPS	$ (370)	$ (444)
Net income attributable to unvested participating securities, diluted EPS	$ (370)	$ (444)