Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 5,208,385	$ 5,055,580	$ 4,979,951	$ 4,926,081	$ 4,846,521	$ 4,784,369	$ 4,979,951	$ 4,784,369
Net unrealized gains (losses)	(7,067)			18,744			94,962	126,862
Amounts reclassified from AOCI	1,395			3,091			4,176	9,100
Other comprehensive income (loss)	(5,672)	1,366	103,444	21,835	58,662	55,465	99,138	135,962
Balance, end of period	8,144,142	5,208,385	5,055,580	4,996,043	4,926,081	4,846,521	8,144,142	4,996,043
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	118,327		31,079	21,071		(75,736)	31,079	(75,736)
Net unrealized gains (losses)	(6,121)			16,889			81,127	113,495
Amounts reclassified from AOCI	1,003			0			1,003	201
Other comprehensive income (loss)	(5,118)			16,889			82,130	113,696
Balance, end of period	113,209	118,327		37,960	21,071		113,209	37,960
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	16,305		3,227	2,184		(12,112)	3,227	(12,112)
Net unrealized gains (losses)	(805)			1,855			13,998	13,367
Amounts reclassified from AOCI	(2,128)			989			(3,853)	3,773
Other comprehensive income (loss)	(2,933)			2,844			10,145	17,140
Balance, end of period	13,372	16,305		5,028	2,184		13,372	5,028
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(269,430)		(273,914)	(285,744)		(288,768)	(273,914)	(288,768)
Net unrealized gains (losses)	(141)			0			(163)	0
Amounts reclassified from AOCI	2,520			2,102			7,026	5,126
Other comprehensive income (loss)	2,379			2,102			6,863	5,126
Balance, end of period	(267,051)	(269,430)		(283,642)	(285,744)		(267,051)	(283,642)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(134,798)		$ (239,608)	(262,489)		$ (376,616)	(239,608)	(376,616)
Balance, end of period	$ (140,470)	$ (134,798)		$ (240,654)	$ (262,489)		$ (140,470)	$ (240,654)