Fair Value Measures - Reconciliations of the changes in assets and liabilities with Level 3 inputs (Details) - CMBS [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 47,215	$ 53,304
Total gains and losses
Unrealized loss included in other comprehensive income (loss), net	(951)	(2,271)
Purchases, issuance, settlements
Return of principal received	(4,388)	(4,077)
Amortization included in net income, net	(902)
Accretion included in net loss, net		259
Ending balance	$ 40,974	$ 47,215