Use of Special Purpose Entities and Variable Interest Entities (Loan Securitization Transactions) (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Securitized debt	$ 363,944	$ 684,420	$ 363,944
Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Unpaid principal balance residential whole loans sold	620,924	1,290,029	620,924
Securitized debt	$ 363,944	$ 684,420	$ 363,944
Weighted average interest rate	3.14%	3.66%	3.14%
Weighted average contractual maturity of Senior Bonds		31 years	32 years
Cash received		$ 802,815	$ 382,845
Debt issuance cost	$ 2,300	3,800	$ 2,300
Spread			300.00%
Debt instrument, coupon step-up period	36 months
Senior Notes | Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Face amount	$ 382,847	802,817	$ 382,847
Proceeds from senior bonds sold with step-up feature		582,800	233,700
Rated and Non-Rated Certificates | Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Face amount	$ 127,001	$ 275,174	$ 127,001