Shareholders' Equity (Details) (USD $)	0 Months Ended	1 Months Ended			3 Months Ended
	Nov. 13, 2012	Mar. 31, 2013	Feb. 28, 2013	Jan. 31, 2013	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Shareholders' Equity
Percentage of dividend on preferred stock	5.50%
Preferred stock, liquidation preference per share	$ 1,000
Components of AOCI
Net unrealized gain on securities available-for-sale		$ 74,201,000			$ 74,201,000				$ 86,526,000
Net unrealized gain on cash flow hedges		21,000			21,000				56,000
Total accumulated other comprehensive income		74,222,000			74,222,000				86,582,000
Components and related tax effects of other comprehensive income
Net unrealized gains arising during the period					(19,810,000)		16,452,000
Reclassification adjustment for net gains included in net income					(887,000)		(394,000)
Non-credit related impairment loss					(492,000)		(2,432,000)
Total securities available for sale					(21,189,000)		13,626,000
Net change on cash flow hedges					(35,000)		(42,000)
Pension liability adjustment							1,085,000
Total other comprehensive income					(21,224,000)		14,669,000
Components and related tax effects of other comprehensive income
Net unrealized gains arising during the period					(8,287,000)		6,882,000
Reclassification adjustment for net gains included in net income, taxes					(371,000)		(165,000)
Non-credit related impairment loss, taxes					(206,000)		(1,018,000)
Total securities available for sale					(8,864,000)		5,699,000
Total other comprehensive income					(8,864,000)		5,699,000
Components and related tax effects of other comprehensive income
Net unrealized (losses) gains arising during the period					(11,523,000)		9,570,000
Reclassification adjustment for net gains/losses included in net income					(516,000)		(229,000)
Non-credit related impairment loss					(286,000)		(1,414,000)
Total securities available for sale					(12,325,000)		7,927,000
Net change on cash flow hedges					(35,000)	[1]	(42,000)	[1]
Pension liability adjustment							1,085,000
Total other comprehensive (loss) income					(12,360,000)		8,970,000
Common shares repurchased
Total number of Shares (or Units) Purchased		22,139	31,125	400	53,664
Average price paid per share (or Unit) (in dollars per share)		$ 56.30	$ 55.63	$ 52.78	$ 55.89
Depositary shares
Shareholders' Equity
Depositary shares of preferred stock issued	7,000,000
Interest of each depositary share in a share of preferred stock public offered	0.025
Depositary shares (in dollars per share)	$ 25.00
Net proceeds from issuance of depositary shares	$ 169,900,000
Preferred stock, redemption period	5 years

[1] See Note 12 for additional information on other comprehensive income related to cash flow hedges.