Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 102,957	$ 116,172	$ 131,961
Other comprehensive income (loss), net of tax:
Net (losses) gains arising during the period	(2,000)	19,398	(13,239)
Less reclassification adjustment for net gains included in net income	(1,446)	(433)	(8,522)
Total	(3,446)	18,965	(21,761)
Non-credit impairment losses on available for sale and held to maturity securities
Net change in non-credit impairment losses on securities	(241)	1,334	3,637
Less reclassification adjustment for credit impairment losses included in net income	(424)	(383)	(268)
Total	(665)	951	3,369
Unrealized gains and losses on derivatives (cash flow hedges)
Net (losses) gains on derivatives arising during the period	(7,239)	(12,147)	2,400
Less reclassification adjustment for net losses included in net income	4,127	3,886	4,005
Total	(3,112)	(8,261)	6,405
Defined benefit pension plan
Net gains (losses) arising during the period	3,444	(16,207)	21,577
Amortization of prior service cost	117	177	1,454
Amortization of net loss	462	132	1,145
Recognition of loss due to curtailment	0	0	468
Total	4,023	(15,898)	24,644
Total other comprehensive (loss) income	(3,200)	(4,243)	12,657
Total comprehensive income	$ 99,757	$ 111,929	$ 144,618