Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net income	$ 279,328	$ 277,977	$ 237,866
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss during the period	(12,450)	103,044	(115,245)
Change in net unrealized gain on securities transferred to held-to-maturity	(33,601)	(35,441)	(35,682)
Reclassification adjustment for net (gains) losses included in net income	(69)	(38)	(1,176)
Total securities available for sale and transferred securities	(46,120)	67,565	(152,103)
Defined-benefit post-retirement benefit plans:
Change in the net actuarial gain/loss	3,118	(34,837)	35,293
Derivatives:
Change in the accumulated gain/loss on effective cash flow hedge derivatives	0	0	(49)
Reclassification adjustments for (gains) losses included in net income:
Interest rate swaps on variable-rate loans	0	(30,604)	(37,380)
Interest rate swap on junior subordinated deferrable interest debentures	0	0	4,064
Total derivatives	0	(30,604)	(33,365)
Other comprehensive income (loss), before tax	(43,002)	2,124	(150,175)
Deferred tax expense (benefit)	(15,051)	744	(52,561)
Other comprehensive income (loss), net of tax	(27,951)	1,380	(97,614)
Comprehensive income	$ 251,377	$ 279,357	$ 140,252