Fair Value Measurements - Summary of Changes in the Fair Values of Level 3 Assets Measured on a Recurring Basis (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Impairment	$ 0	$ 0	$ 0	$ (400,000)
Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	46,400,000		19,200,000
Paid-in-kind dividends			0
Net valuation gain included in earnings	13,500,000		26,000,000.0
Impairment			0
Net valuation gain included in other comprehensive earnings	600,000		15,300,000
Fair value, end of period	60,500,000		60,500,000
Corporate debt securities | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	33,900,000	17,700,000	19,200,000	17,800,000
Paid-in-kind dividends			0	100,000
Net valuation gain included in earnings	0	0	0	0
Impairment			0	(400,000)
Net valuation gain included in other comprehensive earnings	600,000	1,000,000.0	15,300,000	1,200,000
Fair value, end of period	34,500,000	$ 18,700,000	34,500,000	$ 18,700,000
Forward Purchase Agreement | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	12,500,000		0
Paid-in-kind dividends			0
Net valuation gain included in earnings	13,500,000		26,000,000.0
Impairment			0
Net valuation gain included in other comprehensive earnings	0		0
Fair value, end of period	$ 26,000,000.0		$ 26,000,000.0