Borrowings (Details) - USD ($) $ in Thousands | Aug. 01, 2019 | Dec. 10, 2018 | Dec. 07, 2018 | Oct. 22, 2018 | Jul. 09, 2018 | Mar. 08, 2018 | Oct. 11, 2017 | Jun. 08, 2017 | Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2019 |
Debt Instrument [Line Items] | | | | | | | | | | | |
Warehouse Agreement Borrowings | | | | | | | | | $ 279,064 | | $ 60,910 |
Secured Debt, Fair Value | | | | | | | | | 999,113 | | 1,129,202 |
Asset-backed notes at amortized cost | | | | | | | | | 199,602 | | 359,111 |
Line of Credit [Member] | Secured Warehouse Facility [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Warehouse Agreement Borrowings | | | | | | | | | 279,064 | | 60,910 |
Line of Credit Facility, Maximum Borrowing Capacity | | | | | | | | | $ 400,000 | | $ 400,000 |
Line of Credit Facility, Expiration Date | | Oct. 1, 2021 | | | | | | | | | |
Debt Instrument, Description of Variable Rate Basis | | | | | | | | | LIBOR (minimum of 0.00%) + 2.45% | | LIBOR (minimum of 0.00%) + 2.45% |
Line of Credit Facility, Covenant Compliance | | | | | | | | | Yes | Yes | |
Secured Debt [Member] | Asset-backed Notes Series 2019-A [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Covenant Compliance | | | | | | | | | Yes | Yes | |
Secured Debt [Member] | Asset-Backed Notes Series 2018-D [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Covenant Compliance | | | | | | | | | Yes | Yes | |
Secured Debt [Member] | Asset-Backed Notes Series 2018-C [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Covenant Compliance | | | | | | | | | Yes | Yes | |
Secured Debt [Member] | Asset-Backed Notes Series 2018-B [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Covenant Compliance | | | | | | | | | Yes | Yes | |
Secured Debt [Member] | Asset-Backed Notes Series 2018-A [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Covenant Compliance | | | | | | | | | Yes | Yes | |
Secured Debt [Member] | Asset-Backed Notes Series 2017-B [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Covenant Compliance | | | | | | | | | Yes | Yes | |
Secured Debt [Member] | Asset-Backed Notes Series 2017-A [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Covenant Compliance | | | | | | | | | | Yes | |
Fair Value [Member] | Secured Debt [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Face Amount | | | | | | | | | $ 1,154,419 | | $ 1,154,419 |
Secured Debt, Fair Value | | | | | | | | | 999,113 | | 1,129,202 |
Fair Value [Member] | Secured Debt [Member] | Initial Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | 1,226,888 | | 1,226,888 |
Fair Value [Member] | Secured Debt [Member] | Current Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | 1,247,204 | | 1,248,585 |
Fair Value [Member] | Secured Debt [Member] | Asset-backed Notes Series 2019-A [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Face Amount | $ 279,412 | | | | | | | | | | |
Secured Debt, Fair Value | | | | | | | | | 216,255 | | 251,090 |
Debt, Weighted Average Interest Rate | 3.22% | | | | | | | | | | |
Debt Instrument, Term | 3 years | | | | | | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-backed Notes Series 2019-A [Member] | Initial Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | $ 294,118 | | | | | | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-backed Notes Series 2019-A [Member] | Current Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | 299,008 | | 299,813 |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-D [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Face Amount | | | $ 175,002 | | | | | | | | |
Secured Debt, Fair Value | | | | | | | | | 156,504 | | 178,980 |
Debt, Weighted Average Interest Rate | | | 4.50% | | | | | | | | |
Debt Instrument, Term | | | 3 years | | | | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-D [Member] | Initial Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | $ 184,213 | | | | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-D [Member] | Current Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | 187,188 | | 187,447 |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-C [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Face Amount | | | | $ 275,000 | | | | | | | |
Secured Debt, Fair Value | | | | | | | | | 245,320 | | 280,852 |
Debt, Weighted Average Interest Rate | | | | 4.39% | | | | | | | |
Debt Instrument, Term | | | | 3 years | | | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-C [Member] | Initial Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | $ 289,474 | | | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-C [Member] | Current Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | 294,090 | | 294,380 |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-B [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Face Amount | | | | | $ 225,001 | | | | | | |
Secured Debt, Fair Value | | | | | | | | | 195,355 | | 216,306 |
Debt, Weighted Average Interest Rate | | | | | 4.09% | | | | | | |
Debt Instrument, Term | | | | | 3 years | | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-B [Member] | Initial Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | $ 236,854 | | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-B [Member] | Current Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | 240,866 | | 241,000 |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-A [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Face Amount | | | | | | $ 200,004 | | | | | |
Secured Debt, Fair Value | | | | | | | | | 185,679 | | 201,974 |
Debt, Weighted Average Interest Rate | | | | | | 3.83% | | | | | |
Debt Instrument, Term | | | | | | 3 years | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-A [Member] | Initial Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | $ 222,229 | | | | | |
Fair Value [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2018-A [Member] | Current Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | 226,052 | | 225,945 |
Amortized Cost [Member] | Secured Debt [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Face Amount | | | | | | | | | 200,000 | | 360,001 |
Asset-backed notes at amortized cost | | | | | | | | | 199,602 | | 359,111 |
Amortized Cost [Member] | Secured Debt [Member] | Initial Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | 222,231 | | 410,472 |
Amortized Cost [Member] | Secured Debt [Member] | Current Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | 225,867 | | 417,148 |
Amortized Cost [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2017-B [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Face Amount | | | | | | | $ 200,000 | | | | |
Asset-backed notes at amortized cost | | | | | | | | | 199,602 | | 199,413 |
Debt, Weighted Average Interest Rate | | | | | | | 3.51% | | | | |
Debt Instrument, Term | | | | | | | 3 years | | | | |
Amortized Cost [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2017-B [Member] | Initial Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | $ 222,231 | | | | |
Amortized Cost [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2017-B [Member] | Current Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | $ 225,867 | | 225,925 |
Amortized Cost [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2017-A [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Face Amount | | | | | | | | $ 160,001 | | | |
Asset-backed notes at amortized cost | | | | | | | | | | | 159,698 |
Debt, Weighted Average Interest Rate | | | | | | | | 3.36% | | | |
Debt Instrument, Term | | | | | | | | 3 years | | | |
Amortized Cost [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2017-A [Member] | Initial Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | $ 188,241 | | | |
Amortized Cost [Member] | Secured Debt [Member] | Asset-Backed Notes Series 2017-A [Member] | Current Collateral [Member] | | | | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | | | | |
Debt Instrument, Collateral Amount | | | | | | | | | | | $ 191,223 |