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
	Mar. 31, 2020	Mar. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance at beginning of period	$ 55.9
Purchases		$ 54.7
Unrealized (losses) gains included in earnings	(16.5)	1.7
Balance at end of period	39.4
Equity Securities | Level 3 | Long-term investments | Fair Value Measurements Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance at beginning of period	55.9
Purchases		54.7
Unrealized (losses) gains included in earnings	(16.5)	1.7
Balance at end of period	$ 39.4	$ 56.4