Recent Accounting Pronouncements and Other Guidance (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019		Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Recent accounting pronouncements and other guidance	$ 246,873	$ 247,677	$ 76,000	$ 109,222		$ 110,228	$ 106,929	$ 103,419
Allowance for securities held-to-maturity	(220)		220
Cumulative effect adjustment	(2,664,171)	(2,656,211)		(2,370,793)		(2,339,599)	(2,300,573)	(2,054,998)
Senior and subordinated debt	234,563			233,948
Debt Modified Under CARES Act
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Senior and subordinated debt	404,600
Adjustment to apply recent accounting pronouncements
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Recent accounting pronouncements and other guidance				75,757
Cumulative effect adjustment				26,821	[1]			(47,257)	[2]
Unfunded Loan Commitments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Recent accounting pronouncements and other guidance			5,600
PCD
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Recent accounting pronouncements and other guidance	36,885		35,700	3,197
Non-PCD
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Recent accounting pronouncements and other guidance			8,500
Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Recent accounting pronouncements and other guidance			$ 26,000
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment	$ (1,366,986)	$ (1,359,407)		(1,380,612)		$ (1,343,895)	$ (1,304,756)	(1,192,767)
Retained Earnings | Adjustment to apply recent accounting pronouncements
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.