Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 100,231	$ 67,621
Unrealized gains and losses on securities:
Change in unrealized securities gains (losses) arising during period	(78,634)	36,096
Tax effect	18,088	(13,419)
Reclassification adjustment for net gains realized and included in income before income taxes	0	24
Tax effect	0	(9)
Total change in unrealized gains and losses on securities, net of tax	(60,546)	22,692
Change in fair value of cash flow hedges:
Change in unrecognized loss on cash flow hedges	0	0
Tax effect	0	0
Total change in unrecognized loss on cash flow hedges, net of tax	0	0
Change in pension obligation:
Amortization of actuarial losses and prior service cost	3,337	2,500
Tax effect	(768)	(941)
Total change in pension obligation, net of tax	2,569	1,559
Net current period other comprehensive income	(57,977)	24,251
Total comprehensive income	$ 42,254	$ 91,872