Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 117,318	$ 67,064	$ 310,840	$ 269,345
Unrealized (losses) gains on securities available for sale:
Change in unrealized (losses) gains on securities available for sale arising during period	(13,810)	15,752	(9,655)	65,619
Tax effect	3,175	(5,857)	2,221	(24,401)
Reclassification adjustment for gains included in income before income taxes	0	(1,337)	0	(4,664)
Tax effect	0	495	0	1,726
Total change in unrealized (losses) gains on securities available for sale, net of tax	(10,635)	9,053	(7,434)	38,280
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	(109,507)	0
Tax effect	0	0	25,186	0
Reclassification adjustment for accretion of unrealized losses on securities available for sale transferred to held to maturity	6,502	0	10,975	0
Tax effect	(1,495)	0	(2,523)	0
Total change in unrealized losses on securities available for sale transferred to held to maturity, net of tax	5,007	0	(75,869)	0
Change in pension obligation:
Amortization of actuarial losses and prior service cost	3,495	2,330	10,486	7,290
Tax effect	(804)	(864)	(2,412)	(2,702)
Total change in pension obligation, net of tax	2,691	1,466	8,074	4,588
Net current period other comprehensive (loss) income	(2,937)	10,519	(75,229)	42,868
Total comprehensive income	$ 114,381	$ 77,583	$ 235,611	$ 312,213