Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended			3 Months Ended				6 Months Ended
	Jun. 30, 2012	May 31, 2012	Apr. 30, 2012	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011		Dec. 31, 2011
Components of AOCI
Net unrealized gain on securities available-for-sale	$ 82,668			$ 82,668				$ 82,668				$ 73,235
Net unrealized gain on cash flow hedges	139			139				139				222
Pension liability adjustment												(1,085)
Total accumulated other comprehensive income	82,807			82,807				82,807				72,372
Tax effects allocated to each component of OCI
Net unrealized gains arising during the period				6,559		57,168		20,580		40,469
Reclassification adjustment for net gains included in net income				(17)		(1,807)		(411)		(1,895)
Non-credit related impairment loss				(3,951)		(3,838)		(3,951)		(3,838)
Net change on cash flow hedges				(41)		(248)		(83)		(834)
Pension liability adjustment						33		1,085		65
Total other comprehensive income				2,550		51,308		17,220		33,967
Tax effects allocated to each component of OCI
Net unrealized gains arising during the period				2,744		23,914		8,609		16,928
Reclassification adjustment for net gains included in net income, taxes				(7)		(759)		(172)		(796)
Non-credit related impairment loss, taxes				(1,652)		(1,605)		(1,652)		(1,605)
Total other comprehensive income				1,085		21,550		6,785		14,527
Tax effects allocated to each component of OCI
Net unrealized gains arising during the period				3,815		33,254		11,971		23,541
Reclassification adjustment for net gains included in net income				(10)		(1,048)		(239)		(1,099)
Non-credit related impairment loss				(2,299)		(2,233)		(2,299)		(2,233)
Net change on cash flow hedges				(41)	[1]	(248)	[1]	(83)	[1]	(834)	[1]
Pension liability adjustment						33		1,085		65
Total other comprehensive income				$ 1,465		$ 29,758		$ 10,435		$ 19,440
Common shares repurchased
Total number of Shares (or Units) Purchased	1,162	42	131	1,335
Average Price Paid per Share (or Unit) (in dollars per share)	$ 47.97	$ 53.52	$ 52.61	$ 48.60

[1] See Note 12 for additional information on other comprehensive income related to cash flow hedges.