Fair Value Measurements - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net unrealized gain (loss) on securities	$ (116,900,000)	$ 16,500,000	$ (74,100,000)	$ 25,900,000
Securities owned not readily marketable	$ 45,500,000		$ 45,500,000		$ 45,500,000
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other than temporary impairment losses, investments				$ 0