Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) (USD $)	Total	Common Stock	Paid-in Capital	Retained Earnings Substantially Restricted	Accumulated Other Comprehensive Income
Balance, beginning at Dec. 31, 2011	$ 850,227,000	$ 719,000	$ 642,882,000	$ 173,139,000	$ 33,487,000
Balance, beginning, shares at Dec. 31, 2011		71,915,073
Net income	35,314,000			35,314,000
Comprehensive income, accumulated other comprehensive income	10,566,000				10,566,000
Comprehensive income, total	45,880,000
Cash dividends declared	(18,700,000)			(18,700,000)
Stock issuances under stock incentive plans, shares		16,313
Stock issuances under stock incentive plans, value	233,000		233,000
Stock-based compensation and related taxes	(1,459,000)		(1,459,000)
Balance, ending at Jun. 30, 2012	876,181,000	719,000	641,656,000	189,753,000	44,053,000
Balance, ending, shares at Jun. 30, 2012		71,931,386
Balance, beginning at Dec. 31, 2012	900,949,000	719,000	641,737,000	210,531,000	47,962,000
Balance, beginning, shares at Dec. 31, 2012	71,937,222	71,937,222
Net income	43,470,000			43,470,000
Comprehensive income, accumulated other comprehensive income	(24,580,000)				(24,580,000)
Comprehensive income, total	18,890,000
Cash dividends declared	(21,152,000)			(21,152,000)
Stock issuances under stock incentive plans, shares		172,422
Stock issuances under stock incentive plans, value	2,655,000	2,000	2,653,000
Stock issued in connection with acquisitions, shares		1,455,256
Stock issued in connection with acquisitions, value	28,290,000	15,000	28,275,000
Stock-based compensation and related taxes	(630,000)		(630,000)
Balance, ending at Jun. 30, 2013	$ 929,002,000	$ 736,000	$ 672,035,000	$ 232,849,000	$ 23,382,000
Balance, ending, shares at Jun. 30, 2013	73,564,900	73,564,900