Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 134,660	$ 69,316	$ 202,281	$ 121,388
Unrealized gains and losses on securities:
Change in unrealized securities gains (losses) arising during period	13,771	24,176	49,867	92,209
Tax effect	(5,125)	(9,261)	(18,544)	(35,277)
Reclassification adjustment for net gains realized and included in income before income taxes	(3,351)	(12,529)	(3,327)	(17,157)
Tax effect	1,240	4,793	1,231	6,563
Total change in unrealized gains and losses on securities, net of tax	6,535	7,179	29,227	46,338
Change in fair value of cash flow hedges:
Change in unrecognized loss on cash flow hedges	0	729	0	1,429
Tax effect	0	(274)	0	(537)
Total change in unrecognized loss on cash flow hedges, net of tax	0	455	0	892
Change in pension obligation:
Amortization of actuarial losses and prior service cost	2,460	1,882	4,960	3,534
Tax effect	(897)	(719)	(1,838)	(1,351)
Total change in pension obligation, net of tax	1,563	1,163	3,122	2,183
Net current period other comprehensive income	8,098	8,797	32,349	49,413
Total comprehensive income	$ 142,758	$ 78,113	$ 234,630	$ 170,801