Accumulated Other Comprehensive Loss - Changes in AOCI (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2017
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		$ 2,897,509	$ 2,704,703	$ 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		12,308	(5,942)	35,923	(21,460)
Amounts reclassified from accumulated other comprehensive income (loss)		(4,202)	(355)	(5,214)	(52)
Other comprehensive income (loss), net of tax		8,106	(6,297)	30,709	(21,512)
Ending balance		2,953,535	2,750,999	2,953,535	2,750,999
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		(8,260)	(32,501)	(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		29,079	(9,182)	58,660	(30,991)
Amounts reclassified from accumulated other comprehensive income (loss)		(3,286)	(2)	(3,355)	(2)
Other comprehensive income (loss), net of tax		25,793	(9,184)	55,305	(30,993)
Ending balance		17,533	(41,685)	17,533	(41,685)
Defined Benefit Pension Plan, net of tax
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(28,691)	(19,696)	(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	0	0
Amounts reclassified from accumulated other comprehensive income (loss)		76	112	151	224
Other comprehensive income (loss), net of tax		76	112	151	224
Ending balance		(28,615)	(19,584)	(28,615)	(19,584)
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		(1,556)	11,143	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		(16,771)	3,240	(22,737)	9,531
Amounts reclassified from accumulated other comprehensive income (loss)		(992)	(465)	(2,010)	(274)
Other comprehensive income (loss), net of tax		(17,763)	2,775	(24,747)	9,257
Ending balance		(19,319)	13,918	(19,319)	13,918
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(38,507)	(41,054)	(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		$ (30,401)	$ (47,351)	$ (30,401)	$ (47,351)

[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 the changes in accounting policy.