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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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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 gains (losses) included in earnings	$ 11.3	$ 21.0	(5.2)	22.7
Balance at end of period	50.7		50.7
Equity Securities | Level 3 | Long-term investments | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	39.4	56.4	55.9	0.0
Purchases	0.0	0.0	0.0	54.7
Unrealized gains (losses) included in earnings	11.3	21.0	(5.2)	22.7
Balance at end of period	$ 50.7	$ 77.4	$ 50.7	$ 77.4

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- Definition

Amount of restricted investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
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Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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-SubTopic 220
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- Definition

The cash outflow to acquire investments (not to include restricted cash) that are pledged or subject to withdrawal restrictions.

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