Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income	$ 331,151	$ 443,599	$ 454,918
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	427,331	418,556	(182,340)
Change in net unrealized gain on securities transferred to held-to-maturity	(1,256)	(1,292)	(8,818)
Reclassification adjustment for net (gains) losses included in net income	(108,989)	(293)	156
Total securities available for sale and transferred securities before tax amount	317,086	416,971	(191,002)
Defined-benefit post-retirement benefit plans:
Change in the net actuarial gain/loss before tax amount	(11,518)	(3,644)	(7,225)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) before tax amount	5,319	5,623	5,002
Total defined-benefit post-retirement benefit plans	(6,199)	1,979	(2,223)
Reclassification adjustments for (gains) losses included in net income:
Other comprehensive income (loss), before tax	310,887	418,950	(193,225)
Deferred tax expense (benefit)	65,287	87,980	(40,578)
Other comprehensive income (loss), net of tax	245,600	330,970	(152,647)
Comprehensive income	$ 576,751	$ 774,569	$ 302,271