Loans - Held for Investment (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Investments in loans
Total gross loans	$ 7,071,952	$ 5,956,062
Loan loss allowance (loans held-for-investment)	(6,618)	(9,788)	$ (9,424)	$ (6,029)
Carrying Value	7,065,334	5,946,274
Face Amount	7,092,845	5,988,254
Loans with variable rates of interest	$ 5,914,981	5,330,218
Loans with variable rates of interest (as a percent)	92.60%
Weighted average spread of loans (as a percent)	5.20%
1-month LIBOR
Investments in loans
Loans with variable rates of interest	$ 612,994	$ 880,357
Effective variable rate basis (as a percent)	1.2322%	0.7717%
LIBOR floor
Investments in loans
Loans with variable rates of interest	$ 5,301,987	$ 4,449,861
LIBOR floor | Weighted-average
Investments in loans
Effective variable rate basis (as a percent)	0.57%	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)	1.24%	3.00%
Total loans held-for-investment
Investments in loans
Total gross loans	$ 6,388,989	$ 5,857,783
Face Amount	6,416,794	5,890,189
Loans held-for-sale
Investments in loans
Total gross loans	608,624	63,279
Face Amount	$ 601,051	$ 63,065
Weighted Average Life	7 years 7 months 6 days	10 years
Loans held-for-sale | Weighted-average
Investments in loans
Weighted Average Coupon (as a percent)	5.60%	5.30%
Loans transferred as secured borrowings
Investments in loans
Total gross loans	$ 74,339	$ 35,000
Face Amount	$ 75,000	$ 35,000
Weighted Average Life	2 years 6 months	4 months 24 days
Loans transferred as secured borrowings | Weighted-average
Investments in loans
Weighted Average Coupon (as a percent)	5.80%	6.20%
First Mortgages: | Total loans held-for-investment
Investments in loans
Total gross loans	$ 5,527,219	$ 4,865,994
Face Amount	$ 5,549,909	$ 4,881,656
Weighted Average Life	2 years 4 months 24 days	2 years 2 months 12 days
First Mortgages: | Total loans held-for-investment | Weighted-average
Investments in loans
Weighted Average Coupon (as a percent)	6.10%	5.70%
Subordinated mortgages | Total loans held-for-investment
Investments in loans
Total gross loans	$ 222,990	$ 278,032
Face Amount	$ 224,088	$ 293,925
Weighted Average Life	2 years 1 month 6 days	3 years 3 months 18 days
Subordinated mortgages | Total loans held-for-investment | Weighted-average
Investments in loans
Weighted Average Coupon (as a percent)	10.30%	8.90%
Mezzanine Loans
Investments in loans
Carrying Value	$ 1,100,000	$ 964,100
Mezzanine Loans | Total loans held-for-investment
Investments in loans
Total gross loans	615,562	713,757
Face Amount	$ 615,724	$ 714,608
Weighted Average Life	1 year 2 months 12 days	1 year 9 months 18 days
Mezzanine Loans | Total loans held-for-investment | Weighted-average
Investments in loans
Weighted Average Coupon (as a percent)	10.60%	9.60%
Other | Total loans held-for-investment
Investments in loans
Total gross loans	$ 23,218
Face Amount	$ 27,073
Weighted Average Life	4 years 3 months 18 days
Other | Total loans held-for-investment | Weighted-average
Investments in loans
Weighted Average Coupon (as a percent)	8.40%