CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 33,922	$ 32,820	$ 65,232	$ 67,351
Other comprehensive income, net of tax:
Net (losses) gains arising during the period	(16,353)	2,891	(17,040)	7,117
Less reclassification adjustment for net gains included in net income	(25)	(699)	(2,324)	(604)
Total	(16,378)	2,192	(19,364)	6,513
Non-credit impairment losses on available for sale securities
Net change in non-credit impairment losses on securities	2,008	4,547	6,751	11,617
Less reclassification adjustment for credit impairment losses included in net income	(43)	304	(109)	114
Total	1,965	4,851	6,642	11,731
Unrealized gains and losses on derivatives (cash flow hedges)
Net gains (losses) on derivatives arising during the period	2,036	(3,069)	1,959	(2,170)
Less reclassification adjustment for net losses included in net income	995	811	2,086	1,619
Total	3,031	(2,258)	4,045	(551)
Defined benefit pension plan
Net gains arising during the period	18,784		18,769
Amortization of prior service cost	133	103	261	206
Amortization of net loss	471	337	936	675
Recognition of loss due to curtailment	468		468
Total	19,856	440	20,434	881
Total other comprehensive income	8,474	5,225	11,757	18,574
Total comprehensive income	$ 42,396	$ 38,045	$ 76,989	$ 85,925