Accumulated Other Comprehensive Loss - Changes in AOCI (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017		Dec. 31, 2016	[2]
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance				$ 2,668,749	$ 2,581,526
Cumulative effect adjustment of change in accounting policy, net of tax impact						$ 1,339	[1]	$ 3,659
Other comprehensive income (loss) before reclassifications		$ (5,942)	$ 3,127	(21,460)	4,908
Amounts reclassified from accumulated other comprehensive income		(355)	709	(52)	999
Other comprehensive income (loss), net of tax		(6,297)	3,836	(21,512)	5,907
Ending balance		2,750,999	2,639,442	2,750,999	2,639,442
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		(32,501)	(12,420)	(10,348)	(14,142)
Cumulative effect adjustment of change in accounting policy, net of tax impact						(344)
Beginning balance, adjusted						(10,692)
Other comprehensive income (loss) before reclassifications		(9,182)	3,593	(30,991)	5,374
Amounts reclassified from accumulated other comprehensive income		(2)	(50)	(2)	(109)
Other comprehensive income (loss), net of tax		(9,184)	3,543	(30,993)	5,265
Ending balance		(41,685)	(8,877)	(41,685)	(8,877)
Defined Benefit Pension Plans
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(19,696)	(25,545)	(19,808)	(25,894)
Cumulative effect adjustment of change in accounting policy, net of tax impact						0
Beginning balance, adjusted						(19,808)
Other comprehensive income (loss) before reclassifications		0	0		0
Amounts reclassified from accumulated other comprehensive income		112	350	224	699
Other comprehensive income (loss), net of tax		112	350	224	699
Ending balance		(19,584)	(25,195)	(19,584)	(25,195)
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		11,143	0	4,658	0
Cumulative effect adjustment of change in accounting policy, net of tax impact						3
Beginning balance, adjusted						4,661
Other comprehensive income (loss) before reclassifications		3,240	(466)	9,531	(466)
Amounts reclassified from accumulated other comprehensive income		(465)	409	(274)	409
Other comprehensive income (loss), net of tax		2,775	(57)	9,257	(57)
Ending balance		13,918	(57)	13,918	(57)
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(41,054)	(37,965)	(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		$ (47,351)	$ (34,129)	$ (47,351)	$ (34,129)

[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.