Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Allocation of total income tax
Income before income taxes	$ 9,089	$ 5,752	$ 5,051	$ 6,439	$ 6,040	$ 6,518	$ 7,508	$ 6,302	$ 6,463	$ 6,822	$ 6,081	$ 7,491	$ 26,331	$ 26,368	$ 26,857
Shareholders' equity for unrealized (loss)/ gain on securities available-for-sale													(12,177)	1,204	7,029
Shareholders' equity for tax benefit for excess of fair value above cost of stock benefit plans													(635)	(403)	(610)
Shareholders' equity for pension adjustment													9,904	6,056	(11,487)
Shareholders' equity for swap fair value adjustment													1,714	246	(1,506)
Goodwill attributable to acquisition														863
Unallocated income tax													25,137	34,334	20,283
Income tax expense (benefit) applicable to income before taxes
Current													24,908	23,073	19,108
Deferred													1,423	3,295	7,749
Total provision for income taxes	9,089	5,752	5,051	6,439	6,040	6,518	7,508	6,302	6,463	6,822	6,081	7,491	26,331	26,368	26,857
Reconciliation of the expected federal statutory income tax rate to the effective rate
Expected tax rate (as a percent)													35.00%	35.00%	35.00%
Tax-exempt interest income (as a percent)													(4.30%)	(5.00%)	(5.40%)
State income tax, net of federal benefit (as a percent)													2.10%	2.80%	2.90%
Bank-owned life insurance (as a percent)													(1.90%)	(1.90%)	(2.30%)
Dividends on stock plans (as a percent)													(1.10%)	(1.30%)	(0.90%)
Low income housing and historic tax credits (as a percent)													(1.60%)	(0.60%)	(0.60%)
Other (as a percent)													0.10%	0.30%	0.80%
Effective tax rate (as a percent)													28.30%	29.30%	29.50%
Deferred tax assets:
Deferred fee income	197				280								197	280
Deferred compensation expense	3,727				3,463								3,727	3,463
Net operating loss carryforwards	208				482								208	482
Bad debts	20,700				20,639								20,700	20,639
Accrued postretirement benefit cost	716				734								716	734
Stock benefit plans	1,379				1,128								1,379	1,128
Writedown of investment securities	5,301				6,032								5,301	6,032
Reserve for uncollected interest	2,477				4,194								2,477	4,194
Accrued expenses	928				890								928	890
Pension and postretirement benefits	2,202				12,106								2,202	12,106
Unrealized loss on the fair value of derivatives	2,813				4,527								2,813	4,527
Marketable securities available for sale	2,079												2,079
Other	758				582								758	582
Total deferred tax assets	43,485				55,057								43,485	55,057
Deferred tax liabilities:
Pension expense	4,827				5,606								4,827	5,606
Marketable securities available for sale					10,098									10,098
Purchase accounting	1,723				1,808								1,723	1,808
Intangible asset	19,438				17,958								19,438	17,958
Mortgage servicing rights	677				1,151								677	1,151
Fixed assets	7,409				7,814								7,409	7,814
Other	474				821								474	821
Total deferred tax liabilities	34,548				45,256								34,548	45,256
Net deferred tax asset/ (liability)	8,937				9,801								8,937	9,801
Valuation allowance for deferred tax assets	$ 0												$ 0