Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 152,788	$ 153,786	$ 300,104	$ 210,955
Unrealized (losses) gains on securities available for sale:
Unrealized gains (losses) on securities available for sale arising during the period	24,598	28,066	(31,903)	172,909
Tax effect	(5,658)	(6,455)	7,337	(39,768)
Reclassification adjustment for realized gains on securities available for sale included in income before income taxes	(15,830)	(13,752)	(25,037)	(33,547)
Tax effect	3,641	3,163	5,759	7,716
Total change in unrealized (losses) gains on securities available for sale, net of tax	6,751	11,022	(43,844)	107,310
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	(785)	0	(1,260)	0
Tax effect	181	0	290	0
Total change in unrealized losses on securities available for sale transferred to held to maturity, net of tax	(604)	0	(970)	0
Change in pension obligation:
Amortization of actuarial losses	7,018	6,398	13,547	12,662
Tax effect	(1,614)	(1,472)	(3,116)	(2,913)
Total change in pension obligation, net of tax	5,404	4,926	10,431	9,749
Net current period other comprehensive (loss) income	11,551	15,948	(34,383)	117,059
Total comprehensive income	$ 164,339	$ 169,734	$ 265,721	$ 328,014