Fair Value Measurements (Details 3) - Level 3 - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Convertible Preferred Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, fair value measurement with unobservable input	$ 7,388	$ 3,294	$ 13,550
Convertible Preferred Securities | Implied based on market prices
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, fair value measurement with unobservable input	7,388
Convertible Preferred Securities | Cost Basis, Plus Interest
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, fair value measurement with unobservable input		$ 3,294
Real Property | Third-Party Appraisal
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, fair value measurement with unobservable input	$ 29,347