Loans - Held for Investment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Investments in loans
Total gross loans	$ 6,615,568	$ 5,956,062
Loan loss allowance (loans held-for-investment)	(9,483)	(9,788)	$ (5,268)	$ (6,029)
Carrying Value	6,606,085	5,946,274
Face Amount	6,643,846	5,988,254
Loans with variable rates of interest	$ 5,716,636	5,330,218
Loans with variable rates of interest (as a percent)	91.60%
Weighted average spread of loans (as a percent)	5.50%
1-month LIBOR
Investments in loans
Loans with variable rates of interest	$ 879,641	$ 880,357
Effective variable rate basis (as a percent)	0.9828%	0.7717%
LIBOR floor
Investments in loans
Loans with variable rates of interest	$ 4,836,995	$ 4,449,861
LIBOR floor | Weighted-average
Investments in loans
Effective variable rate basis (as a percent)	0.39%	0.36%
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	$ 6,240,302	$ 5,857,783
Face Amount	6,272,649	5,890,189
Loans held-for-sale
Investments in loans
Total gross loans	340,266	63,279
Face Amount	$ 336,197	$ 63,065
Weighted Average Coupon (as a percent)	5.30%	5.30%
Weighted Average Life	7 years 2 months 12 days	10 years
Loans transferred as secured borrowings
Investments in loans
Total gross loans	$ 35,000	$ 35,000
Face Amount	$ 35,000	$ 35,000
Weighted Average Coupon (as a percent)	6.20%	6.20%
Weighted Average Life	2 months 12 days	4 months 24 days
First Mortgages: | Total loans held-for-investment
Investments in loans
Total gross loans	$ 5,212,199	$ 4,865,994
Face Amount	$ 5,228,599	$ 4,881,656
Weighted Average Coupon (as a percent)	5.60%	5.70%
Weighted Average Life	2 years 1 month 6 days	2 years 2 months 12 days
Subordinated mortgages | Total loans held-for-investment
Investments in loans
Total gross loans	$ 294,754	$ 278,032
Face Amount	$ 310,085	$ 293,925
Weighted Average Coupon (as a percent)	8.90%	8.90%
Weighted Average Life	3 years 1 month 6 days	3 years 3 months 18 days
Mezzanine Loans
Investments in loans
Carrying Value	$ 1,100	$ 964,100
Mezzanine Loans | Total loans held-for-investment
Investments in loans
Total gross loans	733,349	713,757
Face Amount	$ 733,965	$ 714,608
Weighted Average Coupon (as a percent)	9.70%	9.60%
Weighted Average Life	1 year 7 months 6 days	1 year 9 months 18 days