Investments - Equity securities and other-long term investments at fair value (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 3.4	$ 0.0
Other long-term investments	454.5	4.0
Valued at Level 3
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	0.0
Other long-term investments	328.1	4.0
Fixed income mutual funds
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	1.9
Fixed income mutual funds | Valued at Level 3
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	0.0
Common stocks
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	1.5
Common stocks | Valued at Level 3
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	0.0
Hedge funds and private equity funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	212.3	0.0
Other long-term investments, unfunded commitments	32.1
Hedge funds and private equity funds | Valued at NAV
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	126.2
Hedge funds and private equity funds | Valued at Level 3
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	86.1
Limited liability companies and private equity securities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	242.2	$ 4.0
Other long-term investments, unfunded commitments	$ 13.8