Shareholders' Equity (Details) (USD $)	0 Months Ended	1 Months Ended			3 Months Ended	12 Months Ended
	Nov. 07, 2013	Dec. 31, 2014	Nov. 30, 2014	Oct. 31, 2014	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 07, 2013
Shareholders' Equity
Percentage of dividend on preferred stock	6.75%
Preferred stock, liquidation preference per share (in dollars per share)	$ 1,000								$ 1,000
Components of other comprehensive income (loss)
Net unrealized gain (loss) on securities available-for-sale		$ (7,070,000)			$ (7,070,000)	$ (7,070,000)	$ (15,641,000)
Foreign currency translation adjustments						(4,000)
Total accumulated other comprehensive (loss) income		(7,074,000)			(7,074,000)	(7,074,000)	(15,641,000)
Securities available-for-sale:
Net unrealized losses arising during the period						22,701,000	(168,824,000)	24,020,000
Reclassification adjustment for net gains included in net income						(7,572,000)	(6,811,000)	(183,000)
Non-credit related impairment loss						(264,000)		(988,000)
Total securities available for sale						14,865,000	(175,635,000)	22,849,000
Foreign currency translation adjustments						(4,000)
Net change on cash flow hedges							(56,000)	(166,000)
Pension liability adjustment								1,085,000
Total other comprehensive income (loss)						14,861,000	(175,691,000)	23,768,000
Components and related tax effects of other comprehensive income (loss)
Net unrealized gain (losses) arising during the period						9,573,000	(70,619,000)	10,047,000
Reclassification adjustment for net gains included in net income, taxes						(3,168,000)	(2,849,000)	(76,000)
Non-credit related impairment loss, taxes						(111,000)		(413,000)
Total securities available for sale						6,294,000	(73,468,000)	9,558,000
Total other comprehensive income (loss)						6,294,000	(73,468,000)	9,558,000
Components of other comprehensive income (loss), net of tax
Net unrealized (losses) gains arising during the period						13,128,000	(98,205,000)	13,973,000
Reclassification adjustment for net gains/losses included in net income						(4,404,000)	(3,962,000)	(107,000)
Non-credit related impairment loss						(153,000)	0	(575,000)
Total securities available for sale						8,571,000	(102,167,000)	13,291,000
Net change on cash flow hedges							(56,000)	(166,000)
Pension liability adjustment								1,085,000
Total other comprehensive income (loss)						8,567,000	(102,223,000)	14,210,000
Number of shares that may yet be purchased		1,140,400			1,140,400	1,140,400
Common shares repurchased
Total number of Shares (or Units) Purchased		201	286	244	731
Average Price Paid per Share (or Unit) (in dollars per share)		$ 77.91	$ 78.48	$ 70.91	$ 75.80
Depositary shares
Shareholders' Equity
Depositary shares of preferred stock issued	4,000,000								4,000,000
Interest of each depositary share in a share of preferred stock public offered	0.025								0.025
Depositary shares (in dollars per share)	$ 25.00								$ 25.00
Net proceeds from issuance of depositary shares	$ 97,700,000
Preferred stock, redemption period	10 years
Preferred stock, alternate redemption period	90 days
Depositary shares | LIBOR
Shareholders' Equity
Preferred stock, variable rate basis effective for February 7, 2024	three-month Libor
Preferred stock, basis spread effective for February 7, 2024 (as a percent)	4.052%								4.052%