Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 111,359	$ 100,231
Unrealized (losses) gains on securities available for sale:
Change in unrealized (losses) gains on securities available for sale arising during period	28,071	(78,635)
Tax effect	(6,456)	18,088
Total change in unrealized (losses) gains on securities available for sale, net of tax	21,615	(60,547)
Unrealized losses on securities available for sale transferred to held to maturity:
Reclassification adjustment for accretion of unrealized losses on securities available for sale transferred to held to maturity	5,962	0
Tax effect	(1,371)	0
Total change in unrealized losses on securities available for sale transferred to held to maturity, net of tax	4,591	0
Change in pension obligation:
Amortization of actuarial losses and prior service cost	2,755	3,337
Tax effect	(634)	(768)
Total change in pension obligation, net of tax	2,121	2,569
Net current period other comprehensive income	28,327	(57,978)
Total comprehensive income	$ 139,686	$ 42,253