Accumulated Other Comprehensive Income (Loss) - Changes, Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net of tax
Balance, beginning	$ 2,524,839	$ 2,532,551	$ 2,476,485
Net income	284,392	264,394	183,682
Other comprehensive income (loss)	100,446	(15,744)	(8,372)
Balance, ending	2,640,258	2,524,839	2,532,551
Non-credit other-than-temporary impairment losses on securities available for sale	0
Accumulated other comprehensive Income (loss)
Net of tax
Balance, beginning	(132,195)	(96,383)	(88,011)
Other comprehensive income (loss)	100,446	(15,744)	(8,372)
Balance, ending	(31,749)	(132,195)	(96,383)
Accumulated other comprehensive Income (loss) | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)	(20,068)
Pension and other benefits, net change in pension and other benefits
Net of tax
Balance, beginning	(28,379)	(25,946)	(30,237)
Other comprehensive income (loss)	297	2,960	4,291
Balance, ending	(28,082)	(28,379)	(25,946)
Pension and other benefits, net change in pension and other benefits | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)	(5,393)
Investment securities:
Net of tax
Balance, beginning	(103,816)	(74,117)	(59,958)
Other comprehensive income (loss)	100,149	(14,259)	(14,159)
Balance, ending	(3,667)	(103,816)	(74,117)
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	2,715	24,085
Cash flow derivative hedges:
Net of tax
Balance, beginning		3,680	2,184
Other comprehensive income (loss)		(4,445)	1,496
Balance, ending			$ 3,680
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