Investments - Equity securities and other-long term investments at fair value (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 5.0	$ 0.0
Other long-term investments	463.7	4.0
Valued at Level 3
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	0.0
Other long-term investments	322.1	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	3.1
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	223.7	0.0
Other long-term investments, unfunded commitments	33.8
Hedge funds and private equity funds | Valued at NAV
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	141.2
Hedge funds and private equity funds | Valued at Level 3
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	82.5
Limited liability companies and private equity securities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	240.0	$ 4.0
Other long-term investments, unfunded commitments	$ 27.8