Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ 364,149	$ 304,261	$ 279,328
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss during the period	157,016	(175,061)	(12,450)
Change in net unrealized gain on securities transferred to held-to-maturity	(16,193)	(32,207)	(33,601)
Reclassification adjustment for net (gains) losses included in net income	4,941	(14,975)	(69)
Total securities available for sale and transferred securities	145,764	(222,243)	(46,120)
Defined-benefit post-retirement benefit plans:
Change in the net actuarial gain/loss	(597)	1,914	(3,877)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit)	5,429	7,274	6,995
Total defined-benefit post-retirement benefit plans	4,832	9,188	3,118
Reclassification adjustments for (gains) losses included in net income:
Other comprehensive income (loss), before tax	150,596	(213,055)	(43,002)
Deferred tax expense (benefit)	46,461	(74,569)	(15,051)
Other comprehensive income (loss), net of tax	104,135	(138,486)	(27,951)
Comprehensive income	$ 468,284	$ 165,775	$ 251,377