Loans - Held for Investment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Investments in loans
Total gross loans	$ 6,435,659	$ 6,269,546
Loan loss allowance (loans held-for-investment)	(5,268)	(6,029)	$ (6,348)	$ (6,031)
Carrying Value	6,430,391	6,263,517
Face Amount	6,493,001	6,335,023
Loans with variable rates of interest	$ 5,255,860	5,052,055
Loans with variable rates of interest (as a percent)	84.90%
Weighted average spread of loans (as a percent)	5.90%
1-month LIBOR
Investments in loans
Loans with variable rates of interest	$ 555,960	$ 438,641
Effective variable rate basis (as a percent)	0.4373%	0.4295%
LIBOR floor
Investments in loans
Loans with variable rates of interest	$ 4,333,422	$ 4,237,947
LIBOR floor | Weighted-average
Investments in loans
Effective variable rate basis (as a percent)	0.32%	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%
3-month LIBOR
Investments in loans
Loans with variable rates of interest	$ 366,478	$ 375,467
Effective variable rate basis (as a percent)	0.5881%	0.5904%
Total loans held-for-investment
Investments in loans
Total gross loans	$ 6,192,922	$ 5,979,108
Face Amount	6,251,126	6,043,313
Loans held-for-sale
Investments in loans
Total gross loans	154,225	203,865
Face Amount	$ 151,970	$ 203,710
Weighted Average Coupon (as a percent)	5.20%	4.90%
Weighted Average Life	9 years 9 months 18 days	9 years 9 months 18 days
Loans transferred as secured borrowings
Investments in loans
Total gross loans	$ 88,512	$ 86,573
Face Amount	$ 89,905	$ 88,000
Weighted Average Coupon (as a percent)	6.10%	6.10%
Weighted Average Life	2 years 2 months 12 days	2 years 4 months 24 days
First Mortgages: | Total loans held-for-investment
Investments in loans
Total gross loans	$ 5,030,487	$ 4,723,852
Face Amount	$ 5,081,967	$ 4,776,576
Weighted Average Coupon (as a percent)	5.90%	6.00%
Weighted Average Life	2 years 4 months 24 days	2 years 8 months 12 days
Subordinated mortgages | Total loans held-for-investment
Investments in loans
Total gross loans	$ 392,315	$ 392,563
Face Amount	$ 414,875	$ 416,713
Weighted Average Coupon (as a percent)	8.50%	8.50%
Weighted Average Life	3 years 2 months 12 days	3 years 4 months 24 days
Mezzanine Loans
Investments in loans
Carrying Value	$ 1,000,000	$ 930,000
Mezzanine Loans | Total loans held-for-investment
Investments in loans
Total gross loans	770,120	862,693
Face Amount	$ 754,284	$ 850,024
Weighted Average Coupon (as a percent)	9.90%	9.90%
Weighted Average Life	2 years 3 months 18 days	2 years 6 months