Stockholders' Equity (Details) (USD $)	0 Months Ended	3 Months Ended		9 Months Ended
	Jul. 16, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Components of basic and diluted earnings per share
Net income		$ 74,586,000	$ 68,419,000	$ 232,438,000	$ 174,239,000
Weighted average shares outstanding - basic		84,242,000	85,603,000	84,775,000	85,688,000
Dilutive potential shares from stock options					1,000
Weighted average shares outstanding - diluted		84,242,000	85,603,000	84,775,000	85,689,000
Earnings per share:
Earnings per share basic and diluted (in dollars per share)		$ 0.89	$ 0.80	$ 2.74	$ 2.03
Dividends
Dividends accrued, per share (in dollars per share)	$ 0.34			$ 1.02
Dividends to be paid		$ 28,600,000		$ 28,600,000
Common stock repurchases
Shares repurchased in the open market or privately		614,062	316,961	1,522,270	1,187,839
Shares repurchased from employees to cover minimum income tax withholdings		1,962	1,961	428,081	468,739