Fair Value Measurements - Carrying Amounts and Estimated Fair Values by Input Level (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Jan. 31, 2017
Financial assets:
Equity securities	$ 112.1	$ 40.5
Financial liabilities:
Interest rate	5.625%
Senior Notes | 5.625% Notes
Financial liabilities:
Interest rate	5.625%	5.625%	5.625%
Recurring | Level 1
Financial assets:
Cash and cash equivalents	$ 1,873.1	$ 437.4
Equity securities	0.0	0.0
Held-to-maturity securities	0.0	0.0
Promissory notes	0.0	0.0
Financial liabilities:
Other liabilities	0.0	0.0
Recurring | Level 1 | Senior Secured Credit Facilities
Financial liabilities:
Long-term debt	1,713.7	1,930.6
Recurring | Level 1 | Senior Notes | 5.625% Notes
Financial liabilities:
Long-term debt	415.0	426.0
Recurring | Level 1 | Senior Notes | 2.75% Convertible Notes due 2026
Financial liabilities:
Long-term debt	1,057.6
Recurring | Level 1 | Other long-term obligations
Financial liabilities:
Long-term debt	0.0	0.0
Recurring | Level 2
Financial assets:
Cash and cash equivalents	0.0	0.0
Equity securities	112.1	40.5
Held-to-maturity securities	6.7	6.7
Promissory notes	15.4	15.1
Financial liabilities:
Other liabilities	2.8	2.8
Recurring | Level 2 | Senior Secured Credit Facilities
Financial liabilities:
Long-term debt	0.0	0.0
Recurring | Level 2 | Senior Notes | 5.625% Notes
Financial liabilities:
Long-term debt	0.0	0.0
Recurring | Level 2 | Senior Notes | 2.75% Convertible Notes due 2026
Financial liabilities:
Long-term debt	0.0
Recurring | Level 2 | Other long-term obligations
Financial liabilities:
Long-term debt	81.1	89.7
Recurring | Level 3
Financial assets:
Cash and cash equivalents	0.0	0.0
Equity securities	0.0	0.0
Held-to-maturity securities	0.0	0.0
Promissory notes	0.0	0.0
Financial liabilities:
Other liabilities	7.0	17.5
Recurring | Level 3 | Senior Secured Credit Facilities
Financial liabilities:
Long-term debt	0.0	0.0
Recurring | Level 3 | Senior Notes | 5.625% Notes
Financial liabilities:
Long-term debt	0.0	0.0
Recurring | Level 3 | Senior Notes | 2.75% Convertible Notes due 2026
Financial liabilities:
Long-term debt	0.0
Recurring | Level 3 | Other long-term obligations
Financial liabilities:
Long-term debt	0.0	0.0
Recurring | Carrying Amount
Financial assets:
Cash and cash equivalents	1,873.1	437.4
Equity securities	112.1	40.5
Held-to-maturity securities	6.7	6.7
Promissory notes	15.1	15.1
Financial liabilities:
Other liabilities	9.8	20.3
Recurring | Carrying Amount | Senior Secured Credit Facilities
Financial liabilities:
Long-term debt	1,723.7	1,896.5
Recurring | Carrying Amount | Senior Notes | 5.625% Notes
Financial liabilities:
Long-term debt	399.4	399.4
Recurring | Carrying Amount | Senior Notes | 2.75% Convertible Notes due 2026
Financial liabilities:
Long-term debt	236.6
Recurring | Carrying Amount | Other long-term obligations
Financial liabilities:
Long-term debt	80.8	89.2
Recurring | Fair Value
Financial assets:
Cash and cash equivalents	1,873.1	437.4
Equity securities	112.1	40.5
Held-to-maturity securities	6.7	6.7
Promissory notes	15.4	15.1
Financial liabilities:
Other liabilities	9.8	20.3
Recurring | Fair Value | Senior Secured Credit Facilities
Financial liabilities:
Long-term debt	1,713.7	1,930.6
Recurring | Fair Value | Senior Notes | 5.625% Notes
Financial liabilities:
Long-term debt	415.0	426.0
Recurring | Fair Value | Senior Notes | 2.75% Convertible Notes due 2026
Financial liabilities:
Long-term debt	1,057.6
Recurring | Fair Value | Other long-term obligations
Financial liabilities:
Long-term debt	$ 81.1	$ 89.7