Recent Accounting Pronouncements and Other Guidance (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019		Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	$ 247,677	$ 226,701	$ 76,000	$ 109,222		$ 106,929	$ 104,779	$ 103,419
Allowance for securities held-to-maturity	(220)		220
Cumulative effect adjustment	(2,656,211)	(2,435,707)		(2,370,793)		(2,300,573)	(2,159,471)	(2,054,998)
COVID-19, loans modification	1,900,000
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss				75,757
Cumulative effect adjustment				26,821	[1]			(47,257)	[2]
Unfunded Loan Commitments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			5,600
PCD
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	44,434		35,700	3,197
Non-PCD
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			8,500
Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			$ 26,000
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment	$ (1,359,407)	$ (1,357,395)		(1,380,612)		$ (1,304,756)	$ (1,273,245)	(1,192,767)
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment				$ 26,821	[1]			$ (47,257)	[2]

[1] As a result of accounting guidance adopted in the first quarter of 2020, a portion of the increase in allowance for credit losses, net of tax, was recognized as a cumulative-effect adjustment to retained earnings as of January 1, 2020. For further discussion of this guidance, see Note 2, "Recent Accounting Pronouncements and Other Guidance."

[2] As a result of accounting guidance adopted in the first quarter of 2019, the remaining deferred gain on a sale-leaseback transaction was recognized as a cumulative-effect adjustment to retained earnings as of January 1, 2019.