Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 116,172	$ 131,961	$ 143,627
Other comprehensive income (loss), net of tax:
Net gains (losses) arising during the period	19,398	(13,239)	5,682
Less reclassification adjustment for net gains included in net income	(433)	(8,522)	(1,545)
Total	18,965	(21,761)	4,137
Non-credit impairment losses on available for sale and held to maturity securities
Net change in non-credit impairment losses on securities	1,334	3,637	9,942
Less reclassification adjustment for credit impairment losses included in net income	(383)	(268)	2,449
Total	951	3,369	12,391
Unrealized gains and losses on derivatives (cash flow hedges)
Net (losses) gains on derivatives arising during the period	(12,147)	2,400	(3,351)
Less reclassification adjustment for net losses included in net income	3,886	4,005	3,760
Total	(8,261)	6,405	409
Defined benefit pension plan
Net (losses) gains arising during the period	(16,207)	21,577	(6,839)
Amortization of prior service cost	177	1,454	38
Amortization of net loss	132	1,145	1,396
Recognition of loss due to curtailment	0	468	0
Total	(15,898)	24,644	(5,405)
Total other comprehensive (loss) gain	(4,243)	12,657	11,532
Total comprehensive income	$ 111,929	$ 144,618	$ 155,159