Shareholders' Equity (Details) (USD $)	0 Months Ended	1 Months Ended		3 Months Ended		6 Months Ended
	Nov. 07, 2013	Jun. 30, 2014	Apr. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Shareholders' Equity
Percentage of dividend on preferred stock	6.75%
Preferred stock, liquidation preference per share (in dollars per share)	$ 1,000
Net unrealized gains (losses) on securities available-for-sale		$ 2,100,000		$ 2,100,000		$ 2,100,000		$ (15,600,000)
Securities available-for-sale:
Net unrealized gains (losses) arising during the period				16,820,000	(111,213,000)	38,348,000	(131,515,000)
Reclassification adjustment for net gains included in net income				(5,359,000)	(4,785,000)	(7,551,000)	(5,672,000)
Non-credit related impairment loss				(317,000)	(240,000)	(317,000)	(240,000)
Total securities available for sale				11,144,000	(116,238,000)	30,480,000	(137,427,000)
Net change on cash flow hedges					(21,000)		(56,000)
Total other comprehensive income (loss)				11,144,000	(116,259,000)	30,480,000	(137,483,000)
Components and related tax effects of other comprehensive income
Net unrealized gain (losses) arising during the period				7,056,000	(46,520,000)	16,031,000	(55,013,000)
Reclassification adjustment for net gains included in net income, taxes				(2,242,000)	(2,002,000)	(3,159,000)	(2,373,000)
Non-credit related impairment loss, taxes				(133,000)	(100,000)	(133,000)	(100,000)
Total securities available for sale				4,681,000	(48,622,000)	12,739,000	(57,486,000)
Total other comprehensive income (loss)				4,681,000	(48,622,000)	12,739,000	(57,486,000)
Components and related tax effects of other comprehensive income
Net unrealized gain (losses) arising during the period				9,764,000	(64,693,000)	22,317,000	(76,502,000)
Reclassification adjustment for net gains/losses included in net income				(3,117,000)	(2,783,000)	(4,392,000)	(3,299,000)
Non-credit related impairment loss				(184,000)	(140,000)	(184,000)	(140,000)
Total securities available for sale				6,463,000	(67,616,000)	17,741,000	(79,941,000)
Net change on cash flow hedges					(21,000)		(56,000)
Total other comprehensive income (loss)				6,463,000	(67,637,000)	17,741,000	(79,997,000)
Common shares repurchased
Total number of Shares (or Units) Purchased		970	127			1,097
Average Price Paid per Share (or Unit) (in dollars per share)		$ 76.33	$ 75.45			$ 76.23
Depositary shares
Shareholders' Equity
Depositary shares of preferred stock issued	4,000,000
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	$ 97,700,000
Preferred stock, redemption period	10 years
Depositary shares | Maximum
Shareholders' Equity
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%