Shareholders' Equity (Details) (USD $)	0 Months Ended	1 Months Ended			3 Months Ended			9 Months Ended
	Nov. 13, 2012	Sep. 30, 2013	Aug. 31, 2013	Jul. 31, 2013	Sep. 30, 2013	Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Shareholders' Equity
Percentage of dividend on preferred stock	5.50%
Preferred stock, liquidation preference per share (in dollars per share)	$ 1,000
Components of AOCI
Net unrealized (loss) gain on securities available-for-sale		$ (10,355,000)			$ (10,355,000)			$ (10,355,000)				$ 86,526,000
Net unrealized gain on cash flow hedges												56,000
Total accumulated other comprehensive (loss) income		(10,355,000)			(10,355,000)			(10,355,000)				86,582,000
Securities available-for-sale:
Net unrealized (losses) gains arising during the period					(23,334,000)	20,701,000		(155,089,000)		37,330,000
Reclassification adjustment for net gains included in net income					(5,788,000)	(48,000)		(11,460,000)		(459,000)
Non-credit related impairment loss						(1,471,000)				(1,471,000)
Total securities available for sale					(29,122,000)	19,182,000		(166,549,000)		35,400,000
Net change on cash flow hedges						(42,000)		(56,000)		(125,000)
Pension liability adjustment										1,085,000
Total other comprehensive (loss) income					(29,122,000)	19,140,000		(166,605,000)		36,360,000
Components and related tax effects of other comprehensive income
Net unrealized (losses) gains arising during the period					(9,761,000)	8,659,000		(64,874,000)		15,615,000
Reclassification adjustment for net gains included in net income, taxes					(2,421,000)	(20,000)		(4,794,000)		(192,000)
Non-credit related impairment loss, taxes						(615,000)				(615,000)
Total securities available for sale					(12,182,000)	8,024,000		(69,668,000)		14,808,000
Total other comprehensive (loss) income					(12,182,000)	8,024,000		(69,668,000)		14,808,000
Components and related tax effects of other comprehensive income
Net unrealized (losses) gains arising during the period					(13,573,000)	12,042,000		(90,215,000)		21,715,000
Reclassification adjustment for net gains/losses included in net income					(3,367,000)	(28,000)		(6,666,000)		(267,000)
Non-credit related impairment loss						(856,000)				(856,000)
Total securities available for sale					(16,940,000)	11,158,000		(96,881,000)		20,592,000
Net change on cash flow hedges						(42,000)	[1]	(56,000)	[1]	(125,000)	[1]
Pension liability adjustment										1,085,000
Total other comprehensive (loss) income					(16,940,000)	11,116,000		(96,937,000)		21,552,000
Common shares repurchased
Total number of Shares (or Units) Purchased		3,022	12	1,912	4,946
Average price paid per share (or Unit) (in dollars per share)		$ 65.70	$ 65.47	$ 68.13	$ 66.64
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.