Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of basic and diluted earnings per share
Net income	$ 62,920	$ 48,058	$ 67,445	$ 67,113	$ 80,893	$ 74,586	$ 82,988	$ 74,864	$ 245,536	$ 313,331	$ 252,998
Weighted average shares outstanding, basic and diluted (in shares)									83,499,000	84,485,000	85,589,000
Earnings per share:
Earnings per share, basic and diluted (in dollars per share)									$ 2.94	$ 3.71	$ 2.96
Dividends
Dividends accrued, per share (in dollars per share)									1.75	$ 1.45	$ 1.18
Quarterly dividend on common stock (in dollars per share)	$ 0.43								$ 0.46
Dividends to be paid	$ 38,100				$ 36,000				$ 38,100	$ 36,000
Common stock repurchases
Shares repurchased in the open market or privately									1,955,509	2,252,152	1,492,535
Shares repurchased from employees to cover minimum income tax withholdings									432,353	599,340	665,035