Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Current:
Federal	$ 28,634,000	$ 30,041,000	$ 18,985,000
State	2,298,000	3,647,000	2,975,000
Total current	30,932,000	33,688,000	21,960,000
Deferred:
Federal	(2,337,000)	(7,087,000)	3,596,000
State	390,000	(554,000)	38,000
Total deferred	(1,947,000)	(7,641,000)	3,634,000
Total provision	28,985,000	26,047,000	25,594,000
Reasons for the difference between income tax expense and statutory federal income tax
Statutory federal income tax	29,380,000	26,488,000	25,826,000
(Decrease) increase in income taxes resulting from Tax-exempt interest income	(1,219,000)	(1,370,000)	(1,668,000)
(Decrease) increase in income taxes resulting from state taxes, net of federal income tax benefit	1,747,000	2,010,000	1,958,000
(Decrease) increase in income taxes resulting from other reconciling items	(923,000)	(1,081,000)	(522,000)
Total provision	28,985,000	26,047,000	25,594,000
Reasons for the difference between income tax expense rate and statutory federal income tax rate
Statutory federal income tax (as a percent)	35.00%	35.00%	35.00%
(Decrease) increase in income taxes resulting from Tax-exempt interest income (as a percent)	(1.50%)	(1.80%)	(2.30%)
(Decrease) increase in income taxes resulting from state taxes, net of federal income tax benefit (as a percent)	2.10%	2.70%	2.70%
(Decrease) increase in income taxes resulting from other reconciling items (as a percent)	(1.10%)	(1.50%)	(0.70%)
Total (as a percent)	34.50%	34.40%	34.70%
Tax applicable on securities gains	170,000	222,000	530,000
Deferred tax assets:
Reserve for loan and lease losses	32,545,000	32,979,000
Accruals for employee benefits	4,153,000	3,590,000
Other	2,243,000	1,831,000
Total deferred tax assets	38,941,000	38,400,000
Deferred tax liabilities:
Differing depreciable bases in premises and leased equipment	22,296,000	23,795,000
Net unrealized gains on securities available-for-sale	3,956,000	12,087,000
Differing bases in assets related to acquisitions	4,725,000	4,376,000
Mortgage servicing	1,588,000	1,493,000
Capitalized loan costs	816,000	1,440,000
Prepaid expenses	931,000	826,000
Other	956,000	781,000
Total deferred tax liabilities	35,268,000	44,798,000
Net deferred tax asset (liability)	3,673,000	(6,398,000)
Reconciliation of the beginning and ending amounts of unrecognized tax benefits
Balance, at beginning of the period	4,068,000	3,387,000	3,424,000
Additions based on tax positions related to the current year	484,000	704,000	419,000
Additions for tax positions of prior years	1,118,000	1,471,000	1,632,000
Reductions for tax positions of prior years	0	(49,000)	(79,000)
Reductions due to lapse in statute of limitations	(1,059,000)	(1,445,000)	(1,165,000)
Settlements	0	0	(844,000)
Balance at end of the period	4,611,000	4,068,000	3,387,000
Unrecognized tax benefits that would affect the effective tax rate if recognized	2,620,000	2,020,000	1,670,000
Interest and penalties net of tax recognized	140,000	(20,000)	(30,000)
Accrued interest and penalties	690,000	550,000	570,000
Increase in earnings from settlement of unrecognized tax benefits			$ 470,000