Shareholders' Equity (Details) (USD $)	0 Months Ended	1 Months Ended			3 Months Ended
	Nov. 07, 2013	Mar. 31, 2014	Feb. 28, 2014	Jan. 31, 2014	Mar. 31, 2014	Mar. 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 losses on securities available-for-sale		$ 4,400,000			$ 4,400,000	$ 15,600,000
Securities available-for-sale:
Net unrealized gains (losses) arising during the period					21,528,000	(19,810,000)
Reclassification adjustment for net gains included in net income					(2,192,000)	(887,000)
Non-credit related impairment loss						(492,000)
Total securities available for sale					19,336,000	(21,189,000)
Net change on cash flow hedges						(35,000)
Total other comprehensive income (loss)					19,336,000	(21,224,000)
Components and related tax effects of other comprehensive income
Net unrealized gain (losses) arising during the period					8,975,000	(8,287,000)
Reclassification adjustment for net gains included in net income, taxes					(917,000)	(371,000)
Non-credit related impairment loss, taxes						(206,000)
Total securities available for sale					8,058,000	(8,864,000)
Total other comprehensive income (loss)					8,058,000	(8,864,000)
Components and related tax effects of other comprehensive income
Net unrealized gain (losses) arising during the period					12,553,000	(11,523,000)
Reclassification adjustment for net gains/losses included in net income					(1,275,000)	(516,000)
Non-credit related impairment loss						(286,000)
Total securities available for sale					11,278,000	(12,325,000)
Net change on cash flow hedges						(35,000)
Total other comprehensive income (loss)					11,278,000	(12,360,000)
Common shares repurchased
Total number of Shares (or Units) Purchased		22,147	28,787	374	51,308
Average Price Paid per Share (or Unit) (in dollars per share)		$ 75.32	$ 72.46	$ 75.12	$ 73.71
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%