Consolidated Statement of Stockholders Equity (USD $) In Thousands, except Share data	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]
Beginning balance at Dec. 31, 2012	$ 894,928	$ 367	$ 561,963	$ 332,105	$ 493
Beginning balance, shares at Dec. 31, 2012	36,687,785	36,687,785
Net income	170,981			170,981
Other comprehensive loss, net of tax	(23)				(23)
Share-based compensation	6,542		6,542
Excess tax benefit on share-based compensation	481		481
Issuance of shares under incentive plans	1,118	3	1,115
Issuance of shares under incentive plans, shares		269,119
Ending balance at Jun. 30, 2013	$ 1,074,027	$ 370	$ 570,101	$ 503,086	$ 470
Ending balance, shares at Jun. 30, 2013	36,956,904	36,956,904