Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 67,621	$ 52,072
Unrealized gains and losses on securities:
Change in unrealized securities gains (losses) arising during period	36,096	68,033
Tax effect	(13,419)	(26,016)
Reclassification adjustment for net gains realized and included in income before income taxes	24	(4,628)
Tax effect	(9)	1,770
Total change in unrealized gains and losses on securities, net of tax	22,692	39,159
Change in fair value of cash flow hedges:
Change in unrecognized loss on cash flow hedges	0	700
Tax effect	0	(263)
Total change in unrecognized loss on cash flow hedges, net of tax	0	437
Change in pension obligation:
Amortization of actuarial losses and prior service cost	2,500	1,652
Tax effect	(941)	(632)
Total change in pension obligation, net of tax	1,559	1,020
Net current period other comprehensive (loss) income	24,251	40,616
Total comprehensive income	$ 91,872	$ 92,688