Allowance for Credit Loss and Credit Quality - Summary of Delinquencies (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total delinquent balance at amortized cost	$ 2,109,578	$ 3,011,763
Total delinquent principal, percent	6.60%	9.10%
Accrued interest receivable	$ 345,769	$ 415,765
Financial Asset Acquired with Credit Deterioration
Financing Receivable, Past Due [Line Items]
Total delinquent balance at amortized cost	984	1,128
Financial Asset Originated and Financial Asset Acquired with Credit Deterioration
Financing Receivable, Past Due [Line Items]
Accrued interest receivable	49,165	73,794
Consumer Portfolio Segment
Financing Receivable, Past Due [Line Items]
Total delinquent balance at amortized cost	2,108,594	3,010,635
Accrued interest receivable	346,000	416,000
30-59 days past due
Financing Receivable, Past Due [Line Items]
Total delinquent balance at amortized cost	$ 1,410,573	$ 1,972,453
Total delinquent principal, percent	4.40%	6.00%
30-59 days past due \| Financial Asset Acquired with Credit Deterioration
Financing Receivable, Past Due [Line Items]
Total delinquent balance at amortized cost	$ 599	$ 687
30-59 days past due \| Consumer Portfolio Segment
Financing Receivable, Past Due [Line Items]
Total delinquent balance at amortized cost	1,409,974	1,971,766
Over 59 days
Financing Receivable, Past Due [Line Items]
Total delinquent balance at amortized cost	$ 699,005	$ 1,039,310
Total delinquent principal, percent	2.20%	3.10%
Over 59 days \| Financial Asset Acquired with Credit Deterioration
Financing Receivable, Past Due [Line Items]
Total delinquent balance at amortized cost	$ 385	$ 441
Over 59 days \| Consumer Portfolio Segment
Financing Receivable, Past Due [Line Items]
Total delinquent balance at amortized cost	$ 698,620	$ 1,038,869

Santander Consumer USA Holdings Inc. 10-Q/A