Summary of Repurchase Agreements Secured by Commercial Mortgage Loans, Long - 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
Commercial Mortgage Loans \| Long-term Borrowings
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	2,641,642	2,526,942
UPB of Collateral	2,435,380	2,714,464
Carrying Value of Collateral	2,431,600	2,705,661
Amounts Payable under Repurchase Agreements	1,617,336	1,796,014
Net Counterparty Exposure	$ 814,264	$ 909,647
Days to Extended Maturity	1126 days	960 days
Goldman Sachs Bank \| Commercial Mortgage Loans \| Long-term Borrowings
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 750,000	$ 750,000
UPB of Collateral	780,647	890,736
Carrying Value of Collateral	777,782	887,667
Amounts Payable under Repurchase Agreements	372,207	568,012
Net Counterparty Exposure	$ 405,575	$ 319,655
Percent of Stockholders' Equity	30.50%	26.60%
Days to Extended Maturity	323 days	596 days
Wells Fargo Bank \| Commercial Mortgage Loans \| Long-term Borrowings
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 750,000	$ 750,000
UPB of Collateral	600,575	814,886
Carrying Value of Collateral	599,306	811,257
Amounts Payable under Repurchase Agreements	444,777	518,353
Net Counterparty Exposure	$ 154,529	$ 292,904
Percent of Stockholders' Equity	11.60%	24.40%
Days to Extended Maturity	968 days	1241 days
Morgan Stanley Bank \| Commercial Mortgage Loans \| Long-term Borrowings
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 500,000	$ 500,000
UPB of Collateral	337,591	533,707
Carrying Value of Collateral	340,286	531,747
Amounts Payable under Repurchase Agreements	256,139	380,592
Net Counterparty Exposure	$ 84,147	$ 151,155
Percent of Stockholders' Equity	6.30%	12.60%
JP Morgan Chase Bank \| Commercial Mortgage Loans \| Long-term Borrowings
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 428,802	$ 376,942
UPB of Collateral	455,267	382,135
Carrying Value of Collateral	454,381	382,542
Amounts Payable under Repurchase Agreements	337,879	257,484
Net Counterparty Exposure	$ 116,502	$ 125,058
Percent of Stockholders' Equity	8.80%	10.40%
Days to Extended Maturity	1785 days	963 days
US Bank \| Commercial Mortgage Loans \| Long-term Borrowings
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 212,840	$ 150,000
UPB of Collateral	261,300	93,000
Carrying Value of Collateral	259,845	92,448
Amounts Payable under Repurchase Agreements	206,334	71,573
Net Counterparty Exposure	$ 53,511	$ 20,875
Percent of Stockholders' Equity	4.00%	1.70%
Days to Extended Maturity	1835 days	1804 days

Assets sold under agreements to repurchase unpaid principal balance of collateral.

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- Definition

Repurchase agreement counterparty amount at risk percent of stockholders equity.

+ References

No definition available.

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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

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

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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.

+ References

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

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

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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).

+ References

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

+ Details

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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.

+ References

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

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