Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income	$ 443,599	$ 454,918	$ 364,149
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	418,556	(182,340)	157,016
Change in net unrealized gain on securities transferred to held-to-maturity	(1,292)	(8,818)	(16,193)
Reclassification adjustment for net (gains) losses included in net income	(293)	156	4,941
Total securities available for sale and transferred securities before tax amount	416,971	(191,002)	145,764
Defined-benefit post-retirement benefit plans:
Change in the net actuarial gain/loss before tax amount	(3,644)	(7,225)	(597)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) before tax amount	5,623	5,002	5,429
Total defined-benefit post-retirement benefit plans	1,979	(2,223)	4,832
Reclassification adjustments for (gains) losses included in net income:
Other comprehensive income (loss), before tax	418,950	(193,225)	150,596
Deferred tax expense (benefit) related to other comprehensive income	87,980	(40,578)	46,461
Other comprehensive income (loss), net of tax	330,970	(152,647)	104,135
Comprehensive income	$ 774,569	$ 302,271	$ 468,284