CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (USD $) In Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings
Beginning Balance at Dec. 31, 2011	$ 1,408,022	$ 995	$ 1,129,435	$ 277,592
Beginning Balance (in shares) at Dec. 31, 2011		99,528
Net income	31,680			31,680
Issuance of common stock	7		7
Retirement of common stock (in shares)		(98)
Retirement of common stock	(2,391)	(1)	(2,390)
Restricted stock compensation, net of forfeitures (in shares)		(7)
Restricted stock compensation, net of forfeitures	1,576		1,576
Income tax benefit of equity compensation	1,920		1,920
Stock option compensation expense, net of forfeitures	1,056		1,056
Restricted stock grant (in shares)		332
Restricted stock grant		3	(3)
Stock options exercised (in shares)		189
Stock options exercised	1,701	2	1,699
Ending Balance at Mar. 31, 2012	1,443,571	999	1,133,300	309,272
Ending Balance (in shares) at Mar. 31, 2012		99,944
Beginning Balance at Dec. 31, 2012	1,521,620	1,001	1,146,488	374,131
Beginning Balance (in shares) at Dec. 31, 2012		100,105
Net income	181,092			181,092
Issuance of common stock	9		9
Retirement of common stock (in shares)		(143)
Retirement of common stock	(5,430)	(1)	(5,429)
Dividends declared on common stock ($0.53 per share)	(53,844)			(53,844)
Restricted stock compensation, net of forfeitures (in shares)		(7)
Restricted stock compensation, net of forfeitures	2,202		2,202
Income tax benefit of equity compensation	23		23
Stock option compensation expense, net of forfeitures	1,003		1,003
Restricted stock grant (in shares)		298
Restricted stock grant		3	(3)
Stock options exercised (in shares)		849
Stock options exercised	15,227	8	15,219
Ending Balance at Mar. 31, 2013	$ 1,661,902	$ 1,011	$ 1,159,512	$ 501,379
Ending Balance (in shares) at Mar. 31, 2013		101,102