Condensed Consolidated Statements of Changes In Shareholders' Equity (Unaudited) (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
BALANCE, VALUE at Dec. 31, 2011	$ 1,672,413	$ 1,514,913	$ 123,726	$ 33,774
BALANCE, SHARES at Dec. 31, 2011		112,164,891
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	101,891		101,891
Other comprehensive income, net of tax	(9,428)			(9,428)
Comprehensive income	92,463
Stock-based compensation	4,041	4,041
Stock repurchased and retired, shares		(596,000)
Stock repurchased and retired, value	(7,436)	(7,436)
Issuances of common stock under stock plans and related tax benefit, shares		321,068
Issuances of common stock under stock plans and related tax benefit, value	882	882
Cash dividends on common stock	(38,324)		(38,324)
BALANCE, VALUE at Dec. 31, 2012	1,724,039	1,512,400	187,293	24,346
BALANCE, SHARES at Dec. 31, 2012	111,889,959	111,889,959
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	23,361		23,361
Other comprehensive income, net of tax	(2,642)			(2,642)
Comprehensive income	20,719
Stock-based compensation	1,185	1,185
Stock repurchased and retired, shares		(33,549)
Stock repurchased and retired, value	(428)	(428)
Issuances of common stock under stock plans and related tax benefit, shares		104,170
Issuances of common stock under stock plans and related tax benefit, value	40	40
Cash dividends on common stock	(11,292)		(11,292)
BALANCE, VALUE at Mar. 31, 2013	$ 1,734,263	$ 1,513,197	$ 199,362	$ 21,704
BALANCE, SHARES at Mar. 31, 2013	111,960,580	111,960,580