Investments and Fair Value Measurements (Quantitative Information about Level 3 Fair Value Measurements) (Details)	Mar. 31, 2020 $ / shares	Dec. 31, 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	$ 24,789,000
Contingent consideration liability		3,147,000
Nonrecurring nonfinancial assets		0	0
Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration liability		3,147,000	6,304,000
Estimated fair value of the Douglas Elliman reporting unit		271,500,000
Significant Unobservable Inputs (Level 3) | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt		$ 4,999,000	$ 31,424,000
Assumed annual 2019 stock dividend | Significant Unobservable Inputs (Level 3) | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		0.05	0.05
Assumed remaining cash dividends - Q4 2019 and Q1 2020 | Significant Unobservable Inputs (Level 3) | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input | $ / shares		0.40	1.60
Stock price | Significant Unobservable Inputs (Level 3) | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input | $ / shares		13.39	9.73
Convertible trading price (as a percentage of par value) | Significant Unobservable Inputs (Level 3) | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		1.0394	1.0031
Volatility | Significant Unobservable Inputs (Level 3) | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		0.3694	0.2039
Estimated fair value of the Douglas Elliman reporting unit			$ 320,000,000
Risk-free rate | Significant Unobservable Inputs (Level 3) | Monte Carlo simulation model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		0.0161	0.0245
Implied credit spread | Significant Unobservable Inputs (Level 3) | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		0.020	0.085
Implied credit spread | Significant Unobservable Inputs (Level 3) | Discounted cash flow | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		0.010	0.080
Implied credit spread | Significant Unobservable Inputs (Level 3) | Discounted cash flow | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		0.030	0.090
Leverage-adjusted equity volatility of peer firms | Significant Unobservable Inputs (Level 3) | Monte Carlo simulation model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		0.3556	0.3022
Forecast | Assumed remaining cash dividends - Q4 2019 and Q1 2020 | Significant Unobservable Inputs (Level 3) | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input | $ / shares	0.20