CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (USD $) In Thousands	Total	Common Stock	Additional Paid-In Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income
Beginning balance at Dec. 31, 2011	$ 1,643,963	$ 14,335	$ 307,059		$ 1,322,562	$ 7
Beginning balance (in shares) at Dec. 31, 2011		143,352
Net income	184,045				184,045
Net unrealized holding gain on available-for-sale securities	(7)					(7)
Common stock issued for employee stock benefit plans	19,599	50	19,549
Common stock issued for employee stock benefit plans (in shares)		501
Income tax benefit from exercise of stock benefit plans	8,659		8,659
Common stock issued to directors	608	3	605
Common stock issued to directors (in shares)		25
Cash dividends paid	(49,534)				(49,534)
Ending balance at Dec. 31, 2012	1,807,333	14,388	335,872		1,457,073
Ending balance (in shares) at Dec. 31, 2012		143,878
Net income	217,112				217,112
Common stock issued for employee stock benefit plans	33,884	154	33,730
Common stock issued for employee stock benefit plans (in shares)		1,541
Income tax benefit from exercise of stock benefit plans	2,358		2,358
Cash dividends paid	(53,546)				(53,546)
Ending balance at Dec. 31, 2013	2,007,141	14,542	371,960		1,620,639
Ending balance (in shares) at Dec. 31, 2013	145,419	145,419
Net income	206,896				206,896
Common stock issued for employee stock benefit plans	30,449	44	30,405
Common stock issued for employee stock benefit plans (in shares)		442
Purchase of treasury stock	(75,025)			(75,025)
Income tax benefit from exercise of stock benefit plans	3,298		3,298
Common stock issued to directors	320	1	319
Common stock issued to directors (in shares)		10
Cash dividends paid	(59,334)				(59,334)
Ending balance at Dec. 31, 2014	$ 2,113,745	$ 14,587	$ 405,982	$ (75,025)	$ 1,768,201
Ending balance (in shares) at Dec. 31, 2014	143,486	145,871