Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common stock	Additional Paid in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance at Dec. 31, 2008	$ 491,544	$ 23,881	$ 346,916	$ 133,578	$ (12,831)
Comprehensive income:
Net income	10,003			10,003
Other:
Change in net unrealized gains on investment securities available-for-sale, net of tax expense of $42, $140 and $10 in 2009, 2010 and 2011 respectively					79
Reclassification adjustment for realized gain on call of investment security - available-for-sale included in net income, net of tax expense of $6					(11)
Adjustment for pension and other postretirement benefits, net of tax expense of $63, $928 and $6,051 in 2009, 2010 and 2011 respectively					116
Comprehensive income	10,187
Cash dividends declared and paid of$1.18, $0.80 and $0.80 in 2009, 2010 and 2011 respectively	(28,190)			(28,190)
Shares issued - stock options	36	1	35
Shares issued - directors' stock purchase plan	244	9	235
Share-based compensation	490		490
Ending balance at Dec. 31, 2009	474,311	23,891	347,676	115,391	(12,647)
Comprehensive income:
Net income	23,090			23,090
Other:
Change in net unrealized gains on investment securities available-for-sale, net of tax expense of $42, $140 and $10 in 2009, 2010 and 2011 respectively					259
Adjustment for pension and other postretirement benefits, net of tax expense of $63, $928 and $6,051 in 2009, 2010 and 2011 respectively					(1,723)
Comprehensive income	21,626
Cash dividends declared and paid of$1.18, $0.80 and $0.80 in 2009, 2010 and 2011 respectively	(21,243)			(21,243)
Shares issued - stock options	42	1	41
Shares and stock options issued in the acquisition of O.A.K. Financial Corporation	83,697	3,530	80,167
Shares issued - directors' stock purchase plan	250	12	238
Share-based compensation	1,395	6	1,389
Ending balance at Dec. 31, 2010	560,078	27,440	429,511	117,238	(14,111)
Comprehensive income:
Net income	43,050			43,050
Other:
Change in net unrealized gains on investment securities available-for-sale, net of tax expense of $42, $140 and $10 in 2009, 2010 and 2011 respectively					19
Adjustment for pension and other postretirement benefits, net of tax expense of $63, $928 and $6,051 in 2009, 2010 and 2011 respectively					(11,237)
Comprehensive income	31,832
Cash dividends declared and paid of$1.18, $0.80 and $0.80 in 2009, 2010 and 2011 respectively	(21,964)			(21,964)
Shares issued - directors' stock purchase plan	266	12	254
Share-based compensation	1,517	5	1,512
Ending balance at Dec. 31, 2011	$ 571,729	$ 27,457	$ 431,277	$ 138,324	$ (25,329)