Summary of Repurchase Agreements Secured by CMBS, Short-term Borrowings, and Counterparty Concentration (Details) - Short-term Debt - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 2,726,407	$ 2,726,942
UPB of Collateral	2,115,463	2,762,214
Carrying Value of Collateral	2,107,203	2,753,549
Amounts Payable under Repurchase Agreements	1,553,012	1,839,319
Net Counterparty Exposure	$ 554,191	$ 914,230
Days to Extended Maturity	851 days	933 days
CMBS
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 200,000	$ 200,000
UPB of Collateral	47,545	47,750
Carrying Value of Collateral	47,544	47,888
Amounts Payable under Repurchase Agreements	43,077	43,305
Net Counterparty Exposure	$ 4,467	$ 4,583
Days to Extended Maturity	65 days	64 days
Goldman Sachs \| CMBS
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 100,000	$ 100,000
UPB of Collateral	39,127	39,332
Carrying Value of Collateral	38,951	39,213
Amounts Payable under Repurchase Agreements	35,308	35,426
Net Counterparty Exposure	$ 3,643	$ 3,787
Percent of Stockholders' Equity	0.30%	0.30%
Days to Extended Maturity	62 days	61 days
Royal Bank of Canada \| CMBS
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 100,000	$ 100,000
UPB of Collateral	8,418	8,418
Carrying Value of Collateral	8,593	8,675
Amounts Payable under Repurchase Agreements	7,769	7,879
Net Counterparty Exposure	$ 824	$ 796
Percent of Stockholders' Equity	0.10%	0.10%
Days to Extended Maturity	81 days	79 days

Repurchase agreement counterparty amount at risk percent of stockholders equity.

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- Definition

Carrying amount of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets.

+ References

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- Definition

Amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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- Definition

The amount at risk under repurchase agreements is defined as the excess of carrying amount (or market value, if higher than the carrying amount or if there is no carrying amount) of the securities or other assets sold under agreement to repurchase, including accrued interest plus any cash or other assets on deposit to secure the repurchase obligation, over the amount of the repurchase liability (adjusted for accrued interest).

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Weighted average maturity (weighted by amount of the agreement) of repurchase agreements with the counterparty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Carrying value, as of the balance sheet date, of securities sold under agreements to repurchase where the transferor maintains effective control over the assets, accounting for them as secured debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 55
-Paragraph 51
-URI http://asc.fasb.org/extlink&oid=77903054&loc=d3e101761-111710

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