Loans - Held for Investment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments in loans
Total gross loans	$ 5,956,062	$ 6,269,546
Loan loss allowance (loans held-for-investment)	(9,788)	(6,029)	$ (6,031)	$ (3,984)
Carrying Value	5,946,274	6,263,517
Face Amount	5,988,254	6,335,023
Loans with variable rates of interest	$ 5,330,218	5,052,055
Loans with variable rates of interest (as a percent)	91.00%
Weighted average spread of loans (as a percent)	5.50%
1-month LIBOR
Investments in loans
Loans with variable rates of interest	$ 880,357	$ 438,641
Effective variable rate basis (as a percent)	0.7717%	0.4295%
3-month LIBOR GBP
Investments in loans
Loans with variable rates of interest		$ 375,467
Effective variable rate basis (as a percent)		0.5904%
LIBOR floor
Investments in loans
Loans with variable rates of interest	$ 4,449,861	$ 4,237,947
LIBOR floor | Weighted-average
Investments in loans
Effective variable rate basis (as a percent)	0.36%	0.31%
LIBOR floor | Minimum
Investments in loans
Effective variable rate basis (as a percent)	0.15%	0.15%
LIBOR floor | Maximum
Investments in loans
Effective variable rate basis (as a percent)	3.00%	3.00%
Total loans held-for-investment
Investments in loans
Total gross loans	$ 5,857,783	$ 5,979,108
Face Amount	5,890,189	6,043,313
Loans held-for-sale
Investments in loans
Total gross loans	63,279	203,865
Face Amount	$ 63,065	$ 203,710
Weighted Average Coupon (as a percent)	5.30%	4.90%
Weighted Average Life	10 years	9 years 9 months 18 days
Loans transferred as secured borrowings
Investments in loans
Total gross loans	$ 35,000	$ 86,573
Face Amount	$ 35,000	$ 88,000
Weighted Average Coupon (as a percent)	6.20%	6.10%
Weighted Average Life	4 months 24 days	2 years 4 months 24 days
First Mortgages: | Total loans held-for-investment
Investments in loans
Total gross loans	$ 4,865,994	$ 4,723,852
Face Amount	$ 4,881,656	$ 4,776,576
Weighted Average Coupon (as a percent)	5.70%	6.00%
Weighted Average Life	2 years 2 months 12 days	2 years 8 months 12 days
Subordinated mortgages | Total loans held-for-investment
Investments in loans
Total gross loans	$ 278,032	$ 392,563
Face Amount	$ 293,925	$ 416,713
Weighted Average Coupon (as a percent)	8.90%	8.50%
Weighted Average Life	3 years 3 months 18 days	3 years 4 months 24 days
Mezzanine Loans
Investments in loans
Carrying Value	$ 964,100	$ 930,000
Mezzanine Loans | Total loans held-for-investment
Investments in loans
Total gross loans	713,757	862,693
Face Amount	$ 714,608	$ 850,024
Weighted Average Coupon (as a percent)	9.60%	9.90%
Weighted Average Life	1 year 9 months 18 days	2 years 6 months