Comprehensive Income - Reclassifications out of AOCI (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment securities gains, net		$ 0	$ 21,003	$ 22,616	$ 29,961
Interest on debt securities held to maturity		49,981	38,893	130,883	101,701
Interest and fees on loans		644,643	771,245	2,029,886	2,359,500
Interest on FHLB and other borrowings		(14,644)	(32,975)	(57,756)	(104,901)
Net income (loss) before income tax (benefit) expense		179,905	222,844	(2,296,987)	590,126
Income tax (expense) benefit		(13,664)	(39,899)	101,506	(106,014)
Net income (loss)		166,241	182,945	(2,195,481)	484,112
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Unrealized Gains (Losses) on Debt Securities Available for Sale and Transferred to Held to Maturity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment securities gains, net	[1]	0	21,003	22,616	29,961
Interest on debt securities held to maturity	[1]	(2,160)	(2,915)	(6,469)	(7,741)
Net income (loss) before income tax (benefit) expense	[1]	(2,160)	18,088	16,147	22,220
Income tax (expense) benefit	[1]	516	(4,288)	(3,851)	(5,268)
Net income (loss)	[1]	(1,644)	13,800	12,296	16,952
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Accumulated Gains (Losses) on Cash Flow Hedging Instruments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest and fees on loans	[1]	43,671	(295)	82,963	(2,765)
Interest on FHLB and other borrowings	[1]	(832)	(254)	(1,841)	(584)
Net income (loss) before income tax (benefit) expense	[1]	42,839	(549)	81,122	(3,349)
Income tax (expense) benefit	[1]	(10,221)	130	(19,355)	794
Net income (loss)	[1]	32,618	(419)	61,767	(2,555)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Defined Benefit Plan Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net periodic expense	[1],[2]	0	0	(2,301)	(4,089)
Income tax (expense) benefit	[1]	0	0	547	970
Net income (loss)	[1]	0	0	(1,754)	(3,119)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Unamortized Impairment Losses on Debt Securities Held to Maturity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest on debt securities held to maturity	[1]	(134)	(229)	(449)	(885)
Income tax (expense) benefit	[1]	32	54	108	210
Net income (loss)	[1]	$ (102)	$ (175)	$ (341)	$ (675)

[1] Amounts in parentheses indicate debits to the Unaudited Condensed Consolidated Statements of Income.

[2] These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see Note 17, Benefit Plans, in the Notes to the December 31, 2019, Consolidated Financial Statements for additional details).