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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 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Balance at beginning of period     $ 38.2  
Unrealized gain (loss) included in earnings $ 0.0 $ 11.3 0.7 $ (5.2)
Balance at end of period 38.9   38.9  
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 38.9 39.4 38.2 55.9
Unrealized gain (loss) included in earnings 0.0 11.3 0.7 (5.2)
Balance at end of period $ 38.9 $ 50.7 $ 38.9 $ 50.7