Accumulated Other Comprehensive Loss - Changes in AOCI (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 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 before reclassifications		(15,518)	1,780
Amounts reclassified from accumulated other comprehensive income		303	291
Other comprehensive income, net of tax		(15,215)	2,071
Ending balance		2,703,277	2,600,051
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		(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 before reclassifications		(21,809)	1,780
Amounts reclassified from accumulated other comprehensive income		0	(58)
Other comprehensive income, net of tax		(21,809)	1,722
Ending balance		(32,501)	(12,420)
Defined Benefit Pension Plans
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(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 before reclassifications		0	0
Amounts reclassified from accumulated other comprehensive income		112	349
Other comprehensive income, net of tax		112	349
Ending balance		(19,696)	(25,545)
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		4,658	0
Cumulative effect adjustment of change in accounting policy, net of tax impact				3
Beginning balance, adjusted				4,661
Other comprehensive income before reclassifications		6,291	0
Amounts reclassified from accumulated other comprehensive income		191	0
Other comprehensive income, net of tax		6,482	0
Ending balance		11,143	0
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(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		$ (41,054)	$ (37,965)

[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.