Summary of Repurchase Agreements Secured by Commercial Mortgage Loans, CMBS, CRE CLO Investments, Long - term Borrowings, and Counterparty Concentration (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 2,633,883	$ 2,612,840
UPB of Collateral	1,904,186	1,575,146
Carrying Value of Collateral	1,901,116	1,572,321
Amounts Payable under Secured Revolving Repurchase Agreements	1,357,806	1,014,555
Net Counterparty Exposure	$ 543,310	$ 557,766
Days to Extended Maturity	933 days	1125 days
CMBS and CRE CLO Investments
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 260,186	$ 200,000
UPB of Collateral	301,952	38,517
Carrying Value of Collateral	301,312	36,414
Amounts Payable under Secured Revolving Repurchase Agreements	260,970	32,984
Net Counterparty Exposure	$ 40,342	$ 3,430
Days to Extended Maturity	28 days	2 days
Commercial Mortgage Loans And CMBS and CRE CLO Investments
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 2,894,069	$ 2,812,840
UPB of Collateral	2,206,138	1,613,663
Carrying Value of Collateral	2,202,428	1,608,735
Amounts Payable under Secured Revolving Repurchase Agreements	1,618,776	1,047,539
Net Counterparty Exposure	$ 583,652	$ 561,196
Days to Extended Maturity	765 days	1083 days
Morgan Stanley Bank \| Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 500,000	$ 500,000
UPB of Collateral	283,514	244,936
Carrying Value of Collateral	282,512	245,932
Amounts Payable under Secured Revolving Repurchase Agreements	214,487	183,901
Net Counterparty Exposure	$ 68,025	$ 62,031
Percent of Stockholders' Equity	4.70%	4.70%
US Bank \| Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 233,883	$ 212,840
UPB of Collateral	273,137	262,929
Carrying Value of Collateral	271,761	261,916
Amounts Payable under Secured Revolving Repurchase Agreements	216,946	206,422
Net Counterparty Exposure	$ 54,815	$ 55,494
Percent of Stockholders' Equity	3.80%	4.20%
Days to Extended Maturity	1836 days	1743 days
Goldman Sachs \| Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 750,000	$ 750,000
UPB of Collateral	550,082	474,243
Carrying Value of Collateral	549,703	472,797
Amounts Payable under Secured Revolving Repurchase Agreements	353,241	191,705
Net Counterparty Exposure	$ 196,462	$ 281,092
Percent of Stockholders' Equity	13.60%	21.20%
Days to Extended Maturity	141 days	231 days
Goldman Sachs \| CMBS and CRE CLO Investments
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 50,666	$ 100,000
UPB of Collateral	61,051	38,517
Carrying Value of Collateral	59,047	36,414
Amounts Payable under Secured Revolving Repurchase Agreements	51,005	32,984
Net Counterparty Exposure	$ 8,042	$ 3,430
Percent of Stockholders' Equity	0.60%	0.30%
Days to Extended Maturity	26 days	2 days
JP Morgan \| Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 400,000	$ 400,000
UPB of Collateral	279,270	254,026
Carrying Value of Collateral	278,240	253,145
Amounts Payable under Secured Revolving Repurchase Agreements	205,752	185,892
Net Counterparty Exposure	$ 72,488	$ 67,253
Percent of Stockholders' Equity	5.00%	5.10%
Days to Extended Maturity	1603 days	1693 days
JP Morgan \| CMBS and CRE CLO Investments
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 182,587
UPB of Collateral	210,090
Carrying Value of Collateral	211,402
Amounts Payable under Secured Revolving Repurchase Agreements	182,982
Net Counterparty Exposure	$ 28,420
Percent of Stockholders' Equity	2.00%
Days to Extended Maturity	28 days
Wells Fargo \| Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 750,000	$ 750,000
UPB of Collateral	518,183	339,012
Carrying Value of Collateral	518,900	338,531
Amounts Payable under Secured Revolving Repurchase Agreements	367,380	246,635
Net Counterparty Exposure	$ 151,520	$ 91,896
Percent of Stockholders' Equity	10.50%	6.90%
Days to Extended Maturity	786 days	876 days
Wells Fargo \| CMBS and CRE CLO Investments
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 26,933
UPB of Collateral	30,811
Carrying Value of Collateral	30,863
Amounts Payable under Secured Revolving Repurchase Agreements	26,983
Net Counterparty Exposure	$ 3,880
Percent of Stockholders' Equity	0.30%
Days to Extended Maturity	29 days
Royal Bank of Canada \| CMBS and CRE CLO Investments
Repurchase Agreement Counterparty [Line Items]
Commitment Amount		$ 100,000

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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