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Debt (Narrative) (Details) - USD ($) |  |  |  |  |  | 9 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Mar. 07, 2019 | Aug. 06, 2018 | Mar. 30, 2015 | Feb. 21, 2014 | Jan. 22, 2014 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum hedging reserve |  |  |  |  |  |  | $ 1,000,000.0 |  |  |
Unamortized Debt Issuance Costs |  |  |  |  |  |  | $ (24,700,000) |  |  | $ (20,500,000)
Percentage of collections on contributed loans |  |  |  |  |  |  | 6.00% |  |  |
Monthly servicing fee per financing |  |  |  |  |  |  | 6.00% |  |  |
Mortgage note | [1] |  |  |  |  |  | $ 11,500,000 |  |  | 11,900,000
Proceeds from issuance of senior notes |  |  |  |  |  |  | 400,000,000.0 |  | $ 0 |
Debt covenants |  |  |  |  |  |  | 1 |  |  |
Unamortized Discount |  |  |  |  |  |  | (900,000) |  |  | (1,100,000)
Carrying Amount |  |  |  |  |  |  | 4,232,200,000 |  |  | 3,820,900,000
Senior Notes [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Unamortized Debt Issuance Costs |  |  |  |  |  |  | (8,900,000) |  |  | (4,500,000)
Unamortized Discount |  |  |  |  |  |  | (900,000) |  |  | (1,100,000)
Carrying Amount |  |  |  |  |  |  | 940,200,000 |  |  | 544,400,000
Mortgages [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  |  | $ 12,000,000.0 |  |  |  | 12,000,000.0 |  |  |
Unamortized Debt Issuance Costs |  |  |  |  |  |  | 0 |  |  | 0
Close date, secured financings |  |  | Aug. 06, 2018 |  |  |  |  |  |  |
Mortgage note |  |  |  |  |  |  | $ 11,500,000 |  |  | $ 11,900,000
Debt maturity date |  |  | Aug. 06, 2023 |  |  |  | Aug. 06, 2023 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 3.73% |  |  | 3.85%
Unamortized Discount |  |  |  |  |  |  | $ 0 |  |  | $ 0
Carrying Amount |  |  |  |  |  |  | 11,500,000 |  |  | 11,900,000
Revolving Secured Line of Credit [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Line of credit facility |  |  |  |  |  |  | $ 340,000,000.0 |  |  |
Percentage of net book value of loans |  |  |  |  |  |  | 80.00% |  |  |
Unamortized Debt Issuance Costs | [2] |  |  |  |  |  | $ 0 |  |  | 0
Unamortized Discount | [2] |  |  |  |  |  | 0 |  |  | 0
Carrying Amount | [2] |  |  |  |  |  | $ 52,200,000 |  |  | $ 171,900,000
Warehouse Facilities [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of net book value of loans |  |  |  |  |  |  | 80.00% |  |  |
Number of warehouse facilities |  |  |  |  |  |  | 6 |  |  |
Debt facility financing amount |  |  |  |  |  |  | $ 1,225,000,000.0 |  |  |
2026 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  | $ 400,000,000.0 |  |  |  |  | $ 400,000,000.0 |  |  |
Close date, secured financings |  | Mar. 07, 2019 |  |  |  |  |  |  |  |
Senior notes stated interest rate |  | 6.625% |  |  |  |  |  |  |  |
Debt maturity date |  |  |  |  |  |  | Mar. 15, 2026 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 6.625% |  |  | 0.00%
Warehouse Facility II [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3],[4] |  |  |  |  |  | $ 400,000,000.0 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Jul. 12, 2022 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 0.00% |  |  | 0.00%
Warehouse Facility IV [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  |  |  |  | $ 300,000,000.0 | [3],[4] |  | $ 250,000,000.0
Debt maturity date | [3],[5] |  |  |  |  |  | Jul. 26, 2022 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 0.00% |  |  | 0.00%
Warehouse Facility V [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3],[4] |  |  |  |  |  | $ 100,000,000.0 |  |  |
Debt maturity date | [3],[6] |  |  |  |  |  | Aug. 17, 2021 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 0.00% |  |  | 0.00%
Warehouse Facility VI [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 75,000,000.0 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Sep. 30, 2022 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 0.00% |  |  | 0.00%
Warehouse Facility VII [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3],[4] |  |  |  |  |  | $ 150,000,000.0 |  |  |
Debt maturity date | [3],[7] |  |  |  |  |  | Dec. 17, 2020 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 0.00% |  |  | 0.00%
2017 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Senior notes stated interest rate |  |  |  |  | 9.125% |  |  |  |  |
Repayment of senior notes |  |  |  |  | $ 350,000,000.0 |  |  |  |  |
2021 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  |  |  | $ 300,000,000.0 | $ 300,000,000.0 |  |  |
Close date, secured financings |  |  |  |  |  | Jan. 22, 2014 |  |  |  |
Senior notes stated interest rate |  |  |  |  |  | 6.125% |  |  |  |
Debt maturity date |  |  |  |  |  | Feb. 15, 2021 | Feb. 15, 2021 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 6.125% |  |  | 6.125%
2023 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  | $ 250,000,000.0 |  |  | $ 250,000,000.0 |  |  |
Close date, secured financings |  |  |  | Mar. 30, 2015 |  |  |  |  |  |
Senior notes stated interest rate |  |  |  | 7.375% |  |  |  |  |  |
Debt maturity date |  | Mar. 15, 2026 |  | Mar. 15, 2023 |  |  | Mar. 15, 2023 |  |  |
Redemption price, percentage of principal amount redeemed |  |  |  | 99.266% |  |  |  |  |  |
Proceeds from issuance of senior notes |  |  |  | $ 248,200,000 |  |  |  |  |  |
Senior notes yield to maturity |  |  |  | 7.50% |  |  | 7.375% |  |  | 7.375%
Term ABS 2016-1 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 0.00% |  |  | 4.41%
Term ABS 2016-2 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 0.00% |  |  | 3.20%
Term ABS 2016-3 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 350,000,000.0 |  |  |
Debt financing close date |  |  |  |  |  |  | Oct. 27, 2016 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Oct. 15, 2018 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 3.29% |  |  | 2.59%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  |  | $ 437,800,000 |  |  |
Term ABS 2017-1 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 350,000,000.0 |  |  |
Debt financing close date |  |  |  |  |  |  | Feb. 23, 2017 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Feb. 15, 2019 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 2.98% |  |  | 2.78%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  |  | $ 437,800,000 |  |  |
Term ABS 2017-2 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 450,000,000.0 |  |  |
Debt financing close date |  |  |  |  |  |  | Jun. 29, 2017 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Jun. 17, 2019 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 2.77% |  |  | 2.72%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  |  | $ 563,200,000 |  |  |
Term ABS 2017-3 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 350,000,000.0 |  |  |
Debt financing close date |  |  |  |  |  |  | Oct. 26, 2017 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Oct. 15, 2019 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 2.88% |  |  | 2.88%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  |  | $ 437,600,000 |  |  |
Term ABS 2018-1 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 500,000,000.0 |  |  |
Debt financing close date |  |  |  |  |  |  | Feb. 22, 2018 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Feb. 17, 2020 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 3.24% |  |  | 3.24%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  |  | $ 625,100,000 |  |  |
Term ABS 2018-2 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 450,000,000.0 |  |  |
Debt financing close date |  |  |  |  |  |  | May 24, 2018 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | May 15, 2020 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 3.68% |  |  | 3.68%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  |  | $ 562,600,000 |  |  |
Term ABS 2018-3 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 398,300,000 |  |  |
Debt financing close date |  |  |  |  |  |  | Aug. 23, 2018 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Aug. 17, 2020 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 3.72% |  |  | 3.72%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  |  | $ 500,100,000 |  |  |
Term ABS 2019-1 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 402,500,000 |  |  |
Debt financing close date |  |  |  |  |  |  | Feb. 21, 2019 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Feb. 15, 2021 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 3.53% |  |  |
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  |  | $ 503,100,000 |  |  |
Term ABS 2019-2 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [3] |  |  |  |  |  | $ 500,000,000.0 |  |  |
Debt financing close date |  |  |  |  |  |  | Aug. 28, 2019 |  |  |
Debt maturity date | [3],[5] |  |  |  |  |  | Aug. 15, 2022 |  |  |
Senior notes yield to maturity |  |  |  |  |  |  | 3.13% |  |  |
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  |  | $ 625,100,000 |  |  |
LIBOR [Member] | Mortgages [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [3],[4] |  |  |  |  |  | 15000.00% |  |  |
LIBOR [Member] | Revolving Secured Line of Credit [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  |  |  | 18750.00% |  |  |
LIBOR [Member] | Warehouse Facility II [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [3],[4] |  |  |  |  |  | 17500.00% |  |  |
LIBOR [Member] | Warehouse Facility IV [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [3],[4] |  |  |  |  |  | 20000.00% |  |  |
LIBOR [Member] | Warehouse Facility V [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [3],[4] |  |  |  |  |  | 19000.00% |  |  |
LIBOR [Member] | Warehouse Facility VI [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [3] |  |  |  |  |  | 20000.00% |  |  |
Prime Rate [Member] | Revolving Secured Line of Credit [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  |  |  | 8750.00% |  |  |
[1] | Measured at amortized cost with fair value disclosed.
[2] | Excludes deferred debt issuance costs of $3.5 million and $2.9 million as of September 30, 2019 and December 31, 2018, respectively, which are included in other assets.
[3] | Financing made available only to a specified subsidiary of the Company.
[4] | Interest rate cap agreements are in place to limit the exposure to increasing interest rates
[5] | Represents the revolving maturity date.
[6] | Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date and any amounts remaining on August 17, 2023 will be due on that date.
[7] | Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date and any amounts remaining on December 17, 2022 will be due on that date.

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