Summary of Secured Revolving Repurchase Agreements, Senior Secured and Secured Credit Agreements, and Asset-Specific Financings (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Index Rate	one-month LIBOR	Index Rate
Senior Secured and Secured Credit Agreements
Debt Instrument [Line Items]
Commitment Amount	$ 660,000	$ 660,000
Maximum Current Availability	200,204	199,619
Balance Outstanding	459,795	460,381
Principal Balance of Collateral	600,350	663,381
Repurchase Agreements
Debt Instrument [Line Items]
Commitment Amount	2,894,069	2,812,840
Maximum Current Availability	1,279,000	1,768,695
Balance Outstanding	1,615,069	1,044,145
Principal Balance of Collateral	2,206,138	1,613,663
CMBS and CRE CLO Investments
Debt Instrument [Line Items]
Commitment Amount	260,186	200,000
Asset-specific Financing
Debt Instrument [Line Items]
Commitment Amount	32,500	32,500
Balance Outstanding	32,500	32,500
Principal Balance of Collateral	45,000	$ 45,000
Debt Instrument, Interest Rate at 4.8% \| Senior Secured and Secured Credit Agreements
Debt Instrument [Line Items]
Maturity Date		Jul. 12, 2020
Index Rate		1 Month LIBOR
Weighted Average Credit Spread		2.30%
Interest Rate		4.80%
Commitment Amount		$ 160,000
Maximum Current Availability		87,059
Balance Outstanding		72,941
Principal Balance of Collateral		169,134
Asset-specific Financing, Secured Revolving Repurchase Agreements and Senior Secured and Secured Credit Agreements
Debt Instrument [Line Items]
Commitment Amount	3,586,569	3,505,340
Maximum Current Availability	1,479,204	1,968,314
Balance Outstanding	2,107,364	1,537,026
Principal Balance of Collateral	$ 2,851,488	$ 2,322,044
BMO Harris Bank \| Debt Instrument, Interest Rate at 4.0% \| Asset-specific Financing
Debt Instrument [Line Items]
Maturity Date	Apr. 09, 2020	Apr. 09, 2020
Index Rate	1 Month LIBOR	1 Month LIBOR
Weighted Average Credit Spread	2.70%	2.70%
Interest Rate	4.00%	4.00%
Commitment Amount	$ 32,500	$ 32,500
Balance Outstanding	32,500	32,500
Principal Balance of Collateral	$ 45,000	$ 45,000
Goldman Sachs \| Debt Instrument, Interest Rate at 4.5% \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date	Aug. 19, 2019
Index Rate	1 Month LIBOR
Weighted Average Credit Spread	2.20%
Interest Rate	4.70%
Commitment Amount	$ 750,000
Maximum Current Availability	397,738
Balance Outstanding	352,262
Principal Balance of Collateral	$ 550,082
Goldman Sachs \| Debt Instrument, Interest Rate at 4.6% \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date		Aug. 19, 2019
Index Rate		1 Month LIBOR
Weighted Average Credit Spread		2.20%
Interest Rate		4.60%
Commitment Amount		$ 750,000
Maximum Current Availability		558,836
Balance Outstanding		191,164
Principal Balance of Collateral		$ 474,243
Goldman Sachs \| Debt Instrument, Interest Rate at 3.1% \| CMBS and CRE CLO Investments \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date	Apr. 25, 2019
Index Rate	1 Month OIS
Weighted Average Credit Spread	0.60%
Interest Rate	3.10%
Commitment Amount	$ 50,666
Balance Outstanding	50,666
Principal Balance of Collateral	$ 61,051
Goldman Sachs \| Debt Instrument, Interest Rate at 2.9% \| CMBS and CRE CLO Investments \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date		Jan. 02, 2019
Index Rate		1 Month OIS
Weighted Average Credit Spread		0.60%
Interest Rate		2.90%
Commitment Amount		$ 100,000
Maximum Current Availability		67,303
Balance Outstanding		32,697
Principal Balance of Collateral		$ 38,517
Wells Fargo \| Debt Instrument, Interest Rate at 4.2% \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date	May 25, 2019
Index Rate	1 Month LIBOR
Weighted Average Credit Spread	1.70%
Interest Rate	4.20%
Commitment Amount	$ 750,000
Maximum Current Availability	383,024
Balance Outstanding	366,976
Principal Balance of Collateral	$ 518,183
Wells Fargo \| Debt Instrument, Interest Rate at 4.3% \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date		May 25, 2019
Index Rate		1 Month LIBOR
Weighted Average Credit Spread		1.80%
Interest Rate		4.30%
Commitment Amount		$ 750,000
Maximum Current Availability		503,792
Balance Outstanding		246,208
Principal Balance of Collateral		$ 339,012
Wells Fargo \| Debt Instrument, Interest Rate at 3.5% \| CMBS and CRE CLO Investments \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date	Apr. 29, 2019
Index Rate	1 Month LIBOR
Weighted Average Credit Spread	1.00%
Interest Rate	3.50%
Commitment Amount	$ 26,933
Balance Outstanding	26,933
Principal Balance of Collateral	$ 30,811
Morgan Stanley \| Debt Instrument, Interest Rate at 4.7% \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date	May 04, 2019	May 04, 2019
Index Rate	1 Month LIBOR	1 Month LIBOR
Weighted Average Credit Spread	2.20%	2.20%
Interest Rate	4.70%	4.70%
Commitment Amount	$ 500,000	$ 500,000
Maximum Current Availability	286,293	317,493
Balance Outstanding	213,707	182,507
Principal Balance of Collateral	$ 283,514	$ 244,936
JP Morgan \| Debt Instrument, Interest Rate at 4.6% \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date	Aug. 20, 2021	Aug. 20, 2021
Index Rate	1 Month LIBOR	1 Month LIBOR
Weighted Average Credit Spread	2.20%	2.20%
Interest Rate	4.60%	4.60%
Commitment Amount	$ 400,000	$ 400,000
Maximum Current Availability	194,702	214,471
Balance Outstanding	205,298	185,529
Principal Balance of Collateral	$ 279,270	$ 254,026
JP Morgan \| Debt Instrument, Interest Rate at 3.5% \| CMBS and CRE CLO Investments \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date	Apr. 27, 2019
Index Rate	1 Month LIBOR
Weighted Average Credit Spread	1.00%
Interest Rate	3.50%
Commitment Amount	$ 182,587
Balance Outstanding	182,587
Principal Balance of Collateral	$ 210,090
US Bank \| Debt Instrument, Interest Rate at 4.3% \| Repurchase Agreements
Debt Instrument [Line Items]
Maturity Date	Oct. 09, 2021	Oct. 09, 2021
Index Rate	1 Month LIBOR	1 Month LIBOR
Weighted Average Credit Spread	1.80%	1.80%
Interest Rate	4.30%	4.30%
Commitment Amount	$ 233,883	$ 212,840
Maximum Current Availability	17,243	6,800
Balance Outstanding	216,640	206,040
Principal Balance of Collateral	$ 273,137	$ 262,929
Royal Bank of Canada \| CMBS and CRE CLO Investments \| Repurchase Agreements
Debt Instrument [Line Items]
Index Rate	N/A	N/A
Commitment Amount		$ 100,000
Maximum Current Availability		$ 100,000
Bank of America \| Debt Instrument, Interest Rate at 4.2% \| Senior Secured and Secured Credit Agreements
Debt Instrument [Line Items]
Maturity Date	Sep. 29, 2020	Sep. 29, 2020
Index Rate	1 Month LIBOR	1 Month LIBOR
Weighted Average Credit Spread	1.80%	1.90%
Interest Rate	4.30%	4.20%
Commitment Amount	$ 500,000	$ 500,000
Maximum Current Availability	101,560	112,560
Balance Outstanding	398,440	387,440
Principal Balance of Collateral	$ 512,699	$ 494,247
Citibank \| Debt Instrument, Interest Rate at 4.8% \| Senior Secured and Secured Credit Agreements
Debt Instrument [Line Items]
Maturity Date	Jul. 12, 2020
Index Rate	1 Month LIBOR
Weighted Average Credit Spread	2.30%
Interest Rate	4.80%
Commitment Amount	$ 160,000
Maximum Current Availability	98,644
Balance Outstanding	61,355
Principal Balance of Collateral	$ 87,651

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.

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Balance Type:	credit
Period Type:	instant

- Definition

Amount of assets pledged to secure a debt instrument.

+ References

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-Publisher FASB
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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.

+ References

No definition available.

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- Definition

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

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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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

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

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