Accumulated Other Comprehensive Loss - Changes in AOCI (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017		Dec. 31, 2016	[2]
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		$ 2,750,999	$ 2,639,442	$ 2,668,749	$ 2,581,526
Cumulative effect adjustment of change in accounting policy, net of tax impact						$ 1,339	[1]	$ 3,659
Other comprehensive (loss) income before reclassifications		(9,893)	8,720	(31,353)	13,627
Amounts reclassified from accumulated other comprehensive income (loss)		(411)	579	(463)	1,579
Other comprehensive income (loss), net of tax		(10,304)	9,299	(31,816)	15,206
Ending balance		2,788,924	2,673,089	2,788,924	2,673,089
Unrealized gains (losses) on securities available-for-sale, net of tax
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(41,685)	(8,877)	(10,348)	(14,142)
Cumulative effect adjustment of change in accounting policy, net of tax impact						(344)
Beginning balance, adjusted						(10,692)
Other comprehensive (loss) income before reclassifications		(13,141)	895	(44,132)	6,268
Amounts reclassified from accumulated other comprehensive income (loss)		0	(1)	(2)	(109)
Other comprehensive income (loss), net of tax		(13,141)	894	(44,134)	6,159
Ending balance		(54,826)	(7,983)	(54,826)	(7,983)
Defined Benefit Pension Plans
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(19,584)	(25,195)	(19,808)	(25,894)
Cumulative effect adjustment of change in accounting policy, net of tax impact						0
Beginning balance, adjusted						(19,808)
Other comprehensive (loss) income before reclassifications		0	7,305	0	7,305
Amounts reclassified from accumulated other comprehensive income (loss)		112	349	336	1,048
Other comprehensive income (loss), net of tax		112	7,654	336	8,353
Ending balance		(19,472)	(17,541)	(19,472)	(17,541)
Unrealized gains (losses) on cash flow hedges, net of tax
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		13,918	(57)	4,658	0
Cumulative effect adjustment of change in accounting policy, net of tax impact						3
Beginning balance, adjusted						4,661
Other comprehensive (loss) income before reclassifications		3,248	520	12,779	54
Amounts reclassified from accumulated other comprehensive income (loss)		(523)	231	(797)	640
Other comprehensive income (loss), net of tax		2,725	751	11,982	694
Ending balance		16,643	694	16,643	694
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(47,351)	(34,129)	(25,498)	(40,036)
Cumulative effect adjustment of change in accounting policy, net of tax impact	[1]					(341)
Beginning balance, adjusted						$ (25,839)
Ending balance		$ (57,655)	$ (24,830)	$ (57,655)	$ (24,830)

[1] Refer to Note 1, Basis of Presentation and Significant Accounting Policies, Note 3, Investment Securities, Note 5, Other Real Estate Owned and Repossessed Assets and Note 9, Derivative Instruments and Balance Sheet Offsetting, for further details on the changes in accounting policy.

[2] Refer to Note 1, Basis of Presentation and Significant Accounting Policies and Note 7, Loan Servicing Rights for further details on the changes in accounting policy.