Stockholders' Equity (Details) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended
	Apr. 16, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Components of basic and diluted earnings per share
Net income		$ 82,988,000	$ 51,957,000	$ 157,852,000	$ 105,820,000
Weighted average shares outstanding - basic		85,073,000	85,869,000	85,046,000	85,731,000
Dilutive potential shares from stock options					1,000
Weighted average shares outstanding - diluted		85,073,000	85,869,000	85,046,000	85,732,000
Earnings per share:
Earnings per share basic and diluted (in dollars per share)		$ 0.98	$ 0.61	$ 1.86	$ 1.23
Dividends
Dividends accrued, per share (in dollars per share)	$ 0.34			$ 0.68
Dividends to be paid		$ 28,800,000		$ 28,800,000
Common stock repurchases
Shares repurchased in the open market or privately		628,022	789,177	908,208	870,878
Common stock repurchases
Shares repurchased from employees to cover minimum income tax withholdings		423,452	464,177	426,119	466,778