Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pre-tax Amount
Balance at beginning of period	$ (8,644)	$ (74,979)	$ (84,722)	$ (85,048)
Other comprehensive income	(8,410)	17,419	67,668	27,488
Balance at end of period	(17,054)	(57,560)	(17,054)	(57,560)
Income Tax Benefit (Expense)
Balance at beginning of period	3,417	29,612	33,463	33,591
Other comprehensive income	3,322	(6,878)	(26,724)	(10,857)
Balance at end of period	6,739	22,734	6,739	22,734
Net of tax
Balance			2,736,941	2,675,040
Other comprehensive (loss) income	(5,088)	10,541	40,944	16,631
Balance	2,523,963	2,744,340	2,523,963	2,744,340
Accumulated Other Comprehensive Loss
Net of tax
Balance	(5,227)	(45,367)	(51,259)	(51,457)
Other comprehensive (loss) income			40,944	16,631
Balance	(10,315)	(34,826)	(10,315)	(34,826)
Pension and other benefits
Pre-tax Amount
Unrealized net gains (losses) arising during the period			(78)
Other comprehensive income			(78)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period			32
Other comprehensive income			32
Net of tax
Balance	(26,929)	(35,869)	(26,883)	(35,869)
Unrealized net gains (losses) arising during the period			(46)
Other comprehensive (loss) income			(46)
Balance	(26,929)	(35,869)	(26,929)	(35,869)
Investment securities
Pre-tax Amount
Unrealized net gains (losses) arising during the period	(9,925)	22,369	92,590	43,488
Reclassification of net gains to net income	(30)	(4,131)	(25,761)	(14,993)
Other comprehensive income	(9,955)	18,238	66,829	28,495
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	3,920	(8,835)	(36,573)	(17,178)
Reclassification of net gains to net income	12	1,633	10,176	5,923
Other comprehensive income	3,932	(7,202)	(26,397)	(11,255)
Net of tax
Balance	21,349	(9,329)	(25,106)	(15,533)
Unrealized net gains (losses) arising during the period	(6,005)	13,534	56,017	26,310
Reclassification of net gains to net income	(18)	(2,498)	(15,585)	(9,070)
Other comprehensive (loss) income	(6,023)	11,036	40,432	17,240
Balance	15,326	1,707	15,326	1,707
Cash flow derivative hedges
Pre-tax Amount
Unrealized net gains (losses) arising during the period	1,545	(819)	917	(550)
Reclassification of net gains to net income				(457)
Other comprehensive income	1,545	(819)	917	(1,007)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(610)	324	(359)	217
Reclassification of net gains to net income				181
Other comprehensive income	(610)	324	(359)	398
Net of tax
Balance	353	(169)	730	(55)
Unrealized net gains (losses) arising during the period	935	(495)	558	(333)
Reclassification of net gains to net income				(276)
Other comprehensive (loss) income	935	(495)	558	(609)
Balance	$ 1,288	$ (664)	$ 1,288	$ (664)