Loans and Investments (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) borrower loan	Dec. 31, 2017 USD ($) borrower loan	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Loans and Investments
Loans and investments, gross	$ 2,776,422	$ 2,652,538
Unearned revenue	(11,217)	(10,628)
Allowance for loan losses	(63,108)	(62,783)	$ (83,016)	$ (83,712)
Loans and investments, net	$ 2,702,097	$ 2,579,127
Percent of Total	100.00%	100.00%
Loan Count | loan	173	170
Wtd. Avg. Pay Rate (as a percent)	6.57%	6.28%
Wtd. Avg. Remaining Months to Maturity	22 months 18 days	23 months 18 days
Wtd. Avg. First Dollar LTV Ratio (as percent)	5.00%	4.00%
Wtd. Avg. Last Dollar LTV Ratio (as percent)	73.00%	73.00%
Number of loans and investments | loan	8	8
Higher-risk
Loans and Investments
Wtd. Avg. Last Dollar LTV Ratio (as percent)	93.00%	93.00%
Carrying value of loans	$ 126,900	$ 126,500
Total assets | Credit risk concentration
Loans and Investments
Loan Count | loan	41	42
Number of different borrowers | borrower	5	5
Concentration risk, percentage	23.00%	24.00%
Bridge loans
Loans and Investments
Loans and investments, gross	$ 2,502,574	$ 2,422,105
Percent of Total	90.00%	91.00%
Loan Count | loan	153	150
Wtd. Avg. Pay Rate (as a percent)	6.41%	6.10%
Wtd. Avg. Remaining Months to Maturity	19 months 18 days	20 months 27 days
Wtd. Avg. First Dollar LTV Ratio (as percent)	0.00%	0.00%
Wtd. Avg. Last Dollar LTV Ratio (as percent)	72.00%	72.00%
Preferred equity investments
Loans and Investments
Loans and investments, gross	$ 188,872	$ 142,892
Percent of Total	7.00%	6.00%
Loan Count | loan	13	12
Wtd. Avg. Pay Rate (as a percent)	6.77%	6.47%
Wtd. Avg. Remaining Months to Maturity	63 months 15 days	68 months 21 days
Wtd. Avg. First Dollar LTV Ratio (as percent)	58.00%	64.00%
Wtd. Avg. Last Dollar LTV Ratio (as percent)	88.00%	90.00%
Mezzanine loans
Loans and Investments
Loans and investments, gross	$ 84,976	$ 87,541
Percent of Total	3.00%	3.00%
Loan Count | loan	7	8
Wtd. Avg. Pay Rate (as a percent)	10.81%	10.78%
Wtd. Avg. Remaining Months to Maturity	19 months 27 days	24 months 24 days
Wtd. Avg. First Dollar LTV Ratio (as percent)	20.00%	20.00%
Wtd. Avg. Last Dollar LTV Ratio (as percent)	66.00%	63.00%