Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 454,918	$ 364,149	$ 304,261
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	(182,340)	157,016	(175,061)
Change in net unrealized gain on securities transferred to held-to-maturity	(8,818)	(16,193)	(32,207)
Reclassification adjustment for net (gains) losses included in net income	156	4,941	(14,975)
Total securities available for sale and transferred securities before tax amount	(191,002)	145,764	(222,243)
Defined-benefit post-retirement benefit plans:
Change in the net actuarial gain/loss before tax amount	(7,225)	(597)	1,914
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) before tax amount	5,002	5,429	7,274
Total defined-benefit post-retirement benefit plans	(2,223)	4,832	9,188
Reclassification adjustments for (gains) losses included in net income:
Other comprehensive income (loss), before tax	(193,225)	150,596	(213,055)
Deferred tax expense (benefit) related to other comprehensive income	(40,578)	46,461	(74,569)
Other comprehensive income (loss), net of tax	(152,647)	104,135	(138,486)
Comprehensive income	$ 302,271	$ 468,284	$ 165,775