Secured Revolving Repurchase Agreements, Senior Secured and Secured Credit Agreements, Term Loan Facility, and Asset-Specific Financings - Additional Information (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) Agreement	Dec. 31, 2018 USD ($) Agreement
Debt Instrument [Line Items]
Index Rate	one-month LIBOR	Index Rate
Term Loan Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 750,000,000
Debt Instrument, outstanding borrowing	$ 267,700
Term Loan Facility \| Minimum
Debt Instrument [Line Items]
Percentage of individual pledged assets	70.00%
Term Loan Facility \| Maximum
Debt Instrument [Line Items]
Percentage of individual pledged assets	85.00%
Term Loan Facility \| LIBOR
Debt Instrument [Line Items]
Line of credit, spread on variable rate	1.85%
Repurchase Agreements
Debt Instrument [Line Items]
Recourse guarantee percentage	100.00%	100.00%
Number of repurchase agreements \| Agreement	5	5
Debt Instrument, outstanding borrowing	$ 1,615,069,000	$ 1,044,145,000
Repurchase Agreements \| Commercial Mortgage Loans
Debt Instrument [Line Items]
Number of repurchase agreements \| Agreement	53	51
Asset-specific Financing, Secured Revolving Repurchase Agreements and Senior Secured and Secured Credit Agreements
Debt Instrument [Line Items]
Recourse guarantee percentage	25.00%	25.00%
Number of repurchase agreements \| Agreement	4	2
Asset-specific Financing, Secured Revolving Repurchase Agreements and Senior Secured and Secured Credit Agreements \| CRE CLO Investments
Debt Instrument [Line Items]
Number of repurchase agreements \| Agreement	10
Asset-specific Financing, Secured Revolving Repurchase Agreements and Senior Secured and Secured Credit Agreements \| CMBS Investments
Debt Instrument [Line Items]
Number of repurchase agreements \| Agreement	2	2
Bank of America \| Senior Secured Credit Agreement
Debt Instrument [Line Items]
Credit agreement initiation date	Mar. 31, 2019
Line of credit facility, maximum borrowing capacity	$ 500,000,000
Line of credit facility, outstanding amount	$ 398,400,000
Line of credit facility, extended maturity date	Sep. 29, 2022
Citibank \| Secured Credit Agreement
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 160,000
Line of credit facility, outstanding amount	$ 61,400,000
Line of credit facility, initial maturity date	Jul. 12, 2020
Percentage of individual pledged assets	70.00%
Individual pledged assets term	90 days
Citibank \| Secured Credit Agreement \| LIBOR
Debt Instrument [Line Items]
Index Rate	one-month LIBOR
Line of credit, spread on variable rate	2.25%
Holdco \| Repurchase Agreements
Debt Instrument [Line Items]
Recourse guarantee percentage	25.00%
Holdco \| Repurchase Agreements \| CMBS and CRE CLO Investments
Debt Instrument [Line Items]
Recourse guarantee percentage	100.00%
Asset-specific Financing
Debt Instrument [Line Items]
Debt Instrument, outstanding borrowing	$ 32,500,000	$ 32,500,000
Asset-specific Financing \| Holdco \| BMO Harris Bank
Debt Instrument [Line Items]
Guaranteed capped rate of outstanding obligations	25.00%

Assets pledged as collateral term.

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Guaranteed capped rate of outstanding obligations.

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Line of credit facility extended maturity date.

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Number of repurchase agreements.

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Percentage of assets pledged as collateral.

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Recourse guarantee percentage.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

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- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

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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.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

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Date the credit facility first became available, in CCYY-MM-DD format.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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