Accumulated Other Comprehensive Loss - Changes in AOCI (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		$ 2,836,260	$ 2,668,749
Cumulative effect adjustment of change in accounting policy, net of tax impact	[1]			$ 1,339
Other comprehensive income (loss) before reclassifications		23,615	(15,518)
Amounts reclassified from accumulated other comprehensive income (loss)		(1,012)	303
Other comprehensive income (loss), net of tax		22,603	(15,215)
Ending balance		2,897,509	2,704,703
Unrealized gains (losses) on securities carried at fair value, net of tax
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(37,772)	(10,348)
Cumulative effect adjustment of change in accounting policy, net of tax impact				(344)
Beginning balance, adjusted				(10,692)
Other comprehensive income (loss) before reclassifications		29,581	(21,809)
Amounts reclassified from accumulated other comprehensive income (loss)		(69)	0
Other comprehensive income (loss), net of tax		29,512	(21,809)
Ending balance		(8,260)	(32,501)
Defined Benefit Pension Plan
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(28,766)	(19,808)
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
Amounts reclassified from accumulated other comprehensive income (loss)		75	112
Other comprehensive income (loss), net of tax		75	112
Ending balance		(28,691)	(19,696)
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		5,428	4,658
Cumulative effect adjustment of change in accounting policy, net of tax impact				3
Beginning balance, adjusted				4,661
Other comprehensive income (loss) before reclassifications		(5,966)	6,291
Amounts reclassified from accumulated other comprehensive income (loss)		(1,018)	191
Other comprehensive income (loss), net of tax		(6,984)	6,482
Ending balance		(1,556)	11,143
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(61,110)	(25,498)
Cumulative effect adjustment of change in accounting policy, net of tax impact	[1]			(341)
Beginning balance, adjusted				$ (25,839)
Ending balance		$ (38,507)	$ (41,054)

[1] Refer to Note 1, Basis of Presentation and Significant Accounting Policies, Note 6, Other Real Estate Owned and Repossessed Assets and Note 20, Accumulated Other Comprehensive Loss for further details on changes in accounting policy.