Comprehensive Income - Reclassifications out of AOCI (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment securities gains, net		$ 3,477	$ 0	$ 22,616	$ 8,958
Interest on debt securities held to maturity		39,800	33,313	80,902	62,808
Interest and fees on loans		669,767	787,767	1,385,243	1,588,255
Interest on FHLB and other borrowings		(21,936)	(34,300)	(43,112)	(71,926)
Net (loss) income before income tax expense		(234,538)	190,698	(2,476,892)	367,282
Income tax (expense) benefit		110,101	(30,512)	115,170	(66,115)
Net (loss) income		(124,437)	160,186	(2,361,722)	301,167
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]	3,477	0	22,616	8,958
Interest on debt securities held to maturity	[1]	(2,240)	(2,542)	(4,309)	(4,826)
Net (loss) income before income tax expense	[1]	1,237	(2,542)	18,307	4,132
Income tax (expense) benefit	[1]	(294)	603	(4,367)	(980)
Net (loss) income	[1]	943	(1,939)	13,940	3,152
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]	33,833	(1,260)	39,292	(2,470)
Interest on FHLB and other borrowings	[1]	(590)	(161)	(1,009)	(330)
Net (loss) income before income tax expense	[1]	33,243	(1,421)	38,283	(2,800)
Income tax (expense) benefit	[1]	(7,932)	337	(9,134)	664
Net (loss) income	[1]	25,311	(1,084)	29,149	(2,136)
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 (loss) income	[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]	(160)	(174)	(315)	(656)
Income tax (expense) benefit	[1]	39	42	76	156
Net (loss) income	[1]	$ (121)	$ (132)	$ (239)	$ (500)

[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).