Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 119,359	$ 93,291	$ 230,718	$ 193,522
Unrealized (losses) gains on securities available for sale:
Change in unrealized gains on securities available for sale arising during period	30,971	82,789	59,042	4,154
Tax effect	(7,123)	(19,042)	(13,579)	(954)
Reclassification adjustment for gains included in income before income taxes	(5,719)	0	(5,719)	0
Tax effect	1,315	0	1,315	0
Total change in unrealized (losses) gains on securities available for sale, net of tax	19,444	63,747	41,059	3,200
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	(109,507)	0	(109,507)
Tax effect	0	25,186	0	25,186
Reclassification adjustment for accretion of unrealized losses on securities available for sale transferred to held to maturity	5,947	4,473	11,909	4,473
Tax effect	(1,368)	(1,028)	(2,739)	(1,028)
Total change in unrealized losses on securities available for sale transferred to held to maturity, net of tax	4,579	(80,876)	9,170	(80,876)
Change in pension obligation:
Amortization of actuarial losses and prior service cost	2,735	3,654	5,490	6,991
Tax effect	(629)	(840)	(1,263)	(1,608)
Total change in pension obligation, net of tax	2,106	2,814	4,227	5,383
Net current period other comprehensive income	26,129	(14,315)	54,456	(72,293)
Total comprehensive income	$ 145,488	$ 78,976	$ 285,174	$ 121,229