Loan Loss Allowance and Credit Quality - Summary of Delinquencies (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Financing Receivable Recorded Investment Past Due [Line Items]
Principal, current	$ 22,180,768	$ 20,111,640
Principal, 31-60 days past due	1,500,738	2,050,688
Delinquent principal over 60 days	712,030	1,037,013
Total principal	24,393,536	23,199,341
Loans Acquired Individually
Financing Receivable Recorded Investment Past Due [Line Items]
Principal, current	20,933,262	18,653,827
Principal, 31-60 days past due	1,290,394	1,729,139
Delinquent principal over 60 days	602,983	855,315
Total principal	22,826,639	21,238,281
Purchased Receivables Portfolios
Financing Receivable Recorded Investment Past Due [Line Items]
Principal, current	1,247,506	1,457,813
Principal, 31-60 days past due	210,344	321,549
Delinquent principal over 60 days	109,047	181,698
Total principal	1,566,897	1,961,060
Receivables from Dealers Held for Investments
Financing Receivable Recorded Investment Past Due [Line Items]
Principal, current	109,105	95,835
Total principal	109,105	95,835
Unsecured Short Term Consumer Loans
Financing Receivable Recorded Investment Past Due [Line Items]
Principal, current	1,088,013	1,072,316
Principal, 31-60 days past due	39,639	28,102
Delinquent principal over 60 days	90,103	65,360
Total principal	$ 1,217,755	$ 1,165,778