Fair Value Measures - Schedule of assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Derivative assets	$ 627	$ 199
Liabilities:
Derivative liabilities	0	(3,547)
Fair Value, Measurements, Recurring [Member]
Assets:
CMBS	40,974	47,215
Derivative assets	627	199
Total assets	41,601	47,414
Liabilities:
Derivative liabilities		(3,547)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
CMBS	0	0
Derivative assets	0	0
Total assets	0	0
Liabilities:
Derivative liabilities		0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
CMBS	0	0
Derivative assets	627	199
Total assets	627	199
Liabilities:
Derivative liabilities		(3,547)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
CMBS	40,974	47,215
Derivative assets	0	0
Total assets	$ 40,974	47,215
Liabilities:
Derivative liabilities		$ 0