Summary of Repurchase Agreements Secured by CMBS, Short-term Borrowings, and Counterparty Concentration (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 3,610,303	$ 3,376,158
Short-term Debt
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	2,841,642	2,726,942
UPB of Collateral	2,474,090	2,762,214
Carrying Value of Collateral	2,469,068	2,753,549
Amounts Payable under Repurchase Agreements	1,652,385	1,839,319
Net Counterparty Exposure	$ 816,683	$ 914,230
Days to Extended Maturity	1099 days	933 days
CMBS | Short-term Debt
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 200,000	$ 200,000
UPB of Collateral	38,710	47,750
Carrying Value of Collateral	37,468	47,888
Amounts Payable under Repurchase Agreements	35,049	43,305
Net Counterparty Exposure	$ 2,419	$ 4,583
Days to Extended Maturity	64 days	64 days
Goldman Sachs | CMBS | Short-term Debt
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 100,000	$ 100,000
UPB of Collateral	38,710	39,332
Carrying Value of Collateral	37,468	39,213
Amounts Payable under Repurchase Agreements	35,049	35,426
Net Counterparty Exposure	$ 2,419	$ 3,787
Percent of Stockholders' Equity	0.20%	0.30%
Days to Extended Maturity	64 days	61 days
Royal Bank of Canada | CMBS | Short-term Debt
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 100,000	$ 100,000
UPB of Collateral		8,418
Carrying Value of Collateral		8,675
Amounts Payable under Repurchase Agreements		7,879
Net Counterparty Exposure		$ 796
Percent of Stockholders' Equity		0.10%
Days to Extended Maturity		79 days