Investments and Fair Value Measurements (Quantitative Information about Level 3 Fair Value Measurements) (Details) $ in Thousands	Mar. 31, 2020 $ / shares	Dec. 31, 2019 $ / shares	Sep. 30, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt			$ 0	$ 24,789
Fair value of contingent liability			5,204	6,304
Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of contingent liability			5,204	6,304
Estimated fair value of the Douglas Elliman reporting unit			320,000	320,000
Discounted cash flow | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt			$ 11,105	$ 31,424
Discounted cash flow | Significant Unobservable Inputs (Level 3) | Assumed annual stock dividend
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt, measurement input			0.05	0.05
Discounted cash flow | Significant Unobservable Inputs (Level 3) | Assumed annual cash dividend
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt, measurement input | $ / shares				1.60
Discounted cash flow | Significant Unobservable Inputs (Level 3) | Stock price
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt, measurement input | $ / shares			11.91	9.73
Discounted cash flow | Significant Unobservable Inputs (Level 3) | Convertible trading price (as a percentage of par value)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt, measurement input			1.0394	1.0031
Discounted cash flow | Significant Unobservable Inputs (Level 3) | Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt, measurement input			0.4225	0.2039
Discounted cash flow | Significant Unobservable Inputs (Level 3) | Implied credit spread
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt, measurement input			0.0270	0.085
Discounted cash flow | Significant Unobservable Inputs (Level 3) | Implied credit spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt, measurement input			0.0170	0.080
Discounted cash flow | Significant Unobservable Inputs (Level 3) | Implied credit spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt, measurement input			0.0370	0.090
Monte Carlo Simulation Model | Significant Unobservable Inputs (Level 3) | Risk-free rate for a 4-year term
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of contingent liability, measurement input			0.0154	0.0245
Monte Carlo Simulation Model | Significant Unobservable Inputs (Level 3) | Leverage-adjusted equity volatility of peer firms
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of contingent liability, measurement input			0.3391	0.3022
Forecast | Discounted cash flow | Significant Unobservable Inputs (Level 3) | Assumed annual cash dividend
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt, measurement input | $ / shares	0.20	0.40