Shareholders' Equity (Details) (USD $)	1 Months Ended			12 Months Ended
	Dec. 31, 2012	Nov. 30, 2012	Oct. 31, 2012	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Nov. 13, 2012	Jan. 24, 2008
Shareholders' Equity
Depositary shares of preferred stock issued		7,000,000
Interest of each depositary share in a share of preferred stock public offered										2.50%
Percentage of dividend on preferred stock		5.50%
Preferred stock, liquidation preference per share										$ 1,000
Depositary shares (in dollars per share)										$ 25.00
Net proceeds from issuance of depositary shares		$ 169,900,000
Preferred stock, redemption period		5 years
Components of AOCI
Net unrealized gain on securities available-for-sale	86,526,000			86,526,000		73,235,000
Net unrealized gain on cash flow hedges	56,000			56,000		222,000
Pension liability adjustment						(1,085,000)
Total accumulated other comprehensive income	86,582,000			86,582,000		72,372,000
Components and related tax effects of other comprehensive income
Net unrealized gains arising during the period				24,020,000		72,499,000		73,808,000
Reclassification adjustment for net gains included in net income				(183,000)		(4,955,000)		8,914,000
Non-credit related impairment loss				(988,000)		(4,198,000)		(7,482,000)
Net change on cash flow hedges				(166,000)		(962,000)		(6,321,000)
Pension liability adjustment				1,085,000		(368,000)		(663,000)
Total other comprehensive income				23,768,000		62,016,000		68,256,000
Components and related tax effects of other comprehensive income
Net unrealized gains arising during the period				10,047,000		30,326,000		30,874,000
Reclassification adjustment for net gains included in net income, taxes				(76,000)		(2,073,000)		3,744,000
Non-credit related impairment loss, taxes				(413,000)		(1,756,000)		(3,130,000)
Net change on cash flow hedges								(3,134,000)
Total other comprehensive income				9,558,000		26,497,000		28,354,000
Components and related tax effects of other comprehensive income
Net unrealized gains arising during the period				13,973,000		42,173,000		42,934,000
Reclassification adjustment for net gains/losses included in net income				(107,000)		(2,882,000)		5,170,000
Non-credit related impairment loss				(575,000)		(2,442,000)		(4,352,000)
Net change on cash flow hedges				(166,000)	[1]	(962,000)	[1]	(3,187,000)	[1]
Pension liability adjustment				1,085,000		(368,000)		(663,000)
Total other comprehensive income				$ 14,210,000		$ 35,519,000		$ 39,902,000
Number of shares authorized to be repurchased											1,000,000
Number of shares that may yet be purchased	1,140,400			1,140,400
Common shares repurchased
Total number of Shares (or Units) Purchased	988		581	1,569
Average Price Paid per Share (or Unit) (in dollars per share)	$ 50.37		$ 50.28	$ 50.34

[1] See Note 16 for additional information on other comprehensive income related to cash flow hedges.