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	49,614		49,614
Other comprehensive income, net of tax	(24,709)			(24,709)
Comprehensive income	24,905
Stock-based compensation	2,222	2,222
Stock repurchased and retired, shares		(208,009)
Stock repurchased and retired, value	(2,811)	(2,811)
Issuances of common stock under stock plans and related tax benefit, shares		216,670
Issuances of common stock under stock plans and related tax benefit, value	846	846
Cash dividends on common stock	(33,849)		(33,849)
BALANCE, VALUE at Jun. 30, 2013	$ 1,715,352	$ 1,512,657	$ 203,058	$ (363)
BALANCE, SHARES at Jun. 30, 2013	111,898,620	111,898,620