Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 124,779	$ 117,318	$ 355,497	$ 310,840
Unrealized (losses) gains on securities available for sale:
Unrealized gains (losses) on securities available for sale arising during the period	3,932	(13,810)	62,974	(9,656)
Tax effect	(906)	3,175	(14,485)	2,221
Reclassification adjustment for realized gains on securities available for sale included in income before income taxes	(1,136)	0	(6,855)	0
Tax effect	262	0	1,577	0
Total change in unrealized (losses) gains on securities available for sale, net of tax	2,152	(10,635)	43,211	(7,435)
Unrealized losses on securities available for sale transferred to held to maturity:
Unrealized losses on securities available for sale transferred to held to maturity	0	0	0	(109,507)
Tax effect	0	0	0	25,186
Reclassification adjustment for accretion of unrealized losses on securities available for sale transferred to held to maturity	6,095	6,502	18,004	10,975
Tax effect	(1,402)	(1,495)	(4,141)	(2,523)
Total change in unrealized losses on securities available for sale transferred to held to maturity, net of tax	4,693	5,007	13,863	(75,869)
Change in pension obligation:
Amortization of actuarial losses and prior service cost	2,745	3,495	8,235	10,486
Tax effect	(631)	(804)	(1,894)	(2,412)
Total change in pension obligation, net of tax	2,114	2,691	6,341	8,074
Net current period other comprehensive income	8,959	(2,937)	63,415	(75,230)
Total comprehensive income	$ 133,738	$ 114,381	$ 418,912	$ 235,610