Accumulated Other Comprehensive Income (Loss) - Changes, Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net of tax
Balance	$ 2,532,551	$ 2,476,485	$ 2,736,941
Net income	264,394	183,682	230,178
Other comprehensive income (loss)	(15,744)	(8,372)	(36,752)
Balance	2,524,839	2,532,551	2,476,485
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	0	0
ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)	(20,068)
Accumulated other comprehensive loss
Net of tax
Balance	(96,383)	(88,011)	(51,259)
Other comprehensive income (loss)	(15,744)	(8,372)	(36,752)
Balance	(132,195)	(96,383)	(88,011)
Accumulated other comprehensive loss | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)	(20,068)
Pension and other benefits:
Net of tax
Balance	(25,946)	(30,237)	(26,883)
Other comprehensive income (loss)	2,960	4,291	(3,354)
Balance	(28,379)	(25,946)	(30,237)
Pension and other benefits: | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)	(5,393)
Investment securities:
Net of tax
Balance	(74,117)	(59,958)	(25,106)
Other comprehensive income (loss)	(14,259)	(14,159)	(34,852)
Balance	(103,816)	(74,117)	(59,958)
Reclassification of net losses (gains) to net income			(27,277)
Investment securities: | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)	(15,440)
Investment securities, OTTI losses on AFS debt securities, net
Net of tax
Reclassification of net losses (gains) to net income	24,085
Cash flow derivative hedges:
Net of tax
Balance	3,680	2,184	730
Other comprehensive income (loss)	(4,445)	1,496	1,454
Balance		$ 3,680	$ 2,184
Reclassification of net losses (gains) to net income	(7,558)
Cash flow derivative hedges: | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)	$ 765