Fair Value Measurements - Reconciliation of Our Investment in Restricted Equity Securities Measured at Fair Value on Quarterly Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period			$ 55.9
Purchases			0.0	$ 54.7
Unrealized loss included in earnings	$ (7.0)	$ (28.5)	(12.2)	(5.8)
Balance at end of period	43.7		43.7
Equity Securities | Level 3 | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	50.7	77.4	55.9	0.0
Purchases	0.0	0.0	0.0	54.7
Unrealized loss included in earnings	(7.0)	(28.5)	(12.2)	(5.8)
Balance at end of period	$ 43.7	$ 48.9	$ 43.7	$ 48.9