Use of Special Purpose Entities and Variable Interest Entities (Loan Securitization Transaction) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Variable Interest Entity [Line Items]
Outstanding amount of Senior Bonds	$ 627,487		$ 684,420
Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Aggregate fair value	1,290,029		1,290,029
Outstanding amount of Senior Bonds	$ 627,487		$ 684,420
Weighted average interest rate	3.67%		3.66%
Weighted average contractual maturity of Senior Bonds	30 years	31 years
Cash received	$ 802,815	$ 802,815
Debt issuance cost	3,300		$ 3,800
Senior Notes \| Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Principal amount of Securitized debt	$ 802,817		802,817
Proceeds from Senior bond sold with Step up feature			$ 582,800
Debt instrument, basis spread on variable rate	3.00%
Debt instrument, coupon step-up period			36 months
Senior Support Certificates \| Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Principal amount of Securitized debt	$ 275,174		$ 275,174

Debt Instrument, Coupon Step-Up Period

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Financing Receivable, Weighted Average Term to Maturity

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Proceeds From Senior Bonds Sold with Step Up Feature

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Residential Whole Loans, Aggregate Fair Value

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

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Weighted average interest rate of debt outstanding.

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Amount, before accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Secured financing other than securities sold under agreements to repurchase and securities loaned.

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The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

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