Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 491,723	$ 457,371	$ 400,313
OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax [Abstract]
Unrealized gains on securities available for sale arising during the period	155,009	64,644	29,170
Tax effect	(35,652)	(14,868)	(6,709)
Reclassification adjustment for realized gains on securities available for sale included in income before income taxes	(60,253)	(7,115)	(351)
Tax effect	13,858	1,636	81
Unrealized gains on securities available for sale arising during the period, net of tax	72,962	44,297	22,191
Unrealized losses on securities available for sale transferred to held to maturity [Abstract]
Unrealized gains (losses) on securities available for sale transferred from/to held to maturity	5,894	72,512	(109,507)
Tax effect	(1,356)	(16,678)	25,186
Reclassification adjustment for accretion of unrealized (gains) losses on securities available for sale transferred to held to maturity	(495)	19,889	17,106
Tax effect	114	(4,574)	(3,934)
Total change in unrealized gains (losses) on securities available for sale transferred to held to maturity, net of tax	4,157	71,149	(71,149)
Defined benefit pension items:
Actuarial gains (losses) arising during the period	55,023	(20,049)	(32,012)
Tax effect	(12,656)	4,611	7,363
Amortization of actuarial losses and prior service cost	25,324	10,981	13,981
Other Comprehensive Income (Loss), Defined Benefit Plan, Amortization of Gains (Losses) And Prior Service Cost, Tax	5,824	2,525	3,216
Total change from defined benefit plans, net of tax	(61,867)	6,982	13,884
Net current period other comprehensive income	138,986	108,464	(62,842)
Total comprehensive income	$ 630,709	$ 565,835	$ 337,471