Investment securities - Equity securities and Other long-term investments (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Equity securities
Equity securities and Other long-term investments
Cost or amortized cost	$ 178.1	$ 379.2
Gross unrealized gains	40.1	55.6
Gross unrealized (losses)	(69.1)	(37.3)
Net foreign currency gains	0.8	7.7
Fair value	149.9	405.2
Other long-term investments
Equity securities and Other long-term investments
Cost or amortized cost	324.7	315.4
Gross unrealized gains	61.3	49.9
Gross unrealized (losses)	(28.8)	(29.3)
Net foreign currency gains	10.9	10.8
Fair value	368.1	346.8
Other long-term investments | Hedge funds and private equity funds
Equity securities and Other long-term investments
Fair value	289.9	269.0
Other long-term investments | Limited liability companies and private equity securities
Equity securities and Other long-term investments
Fair value	78.2	77.8
Fixed income mutual funds
Equity securities and Other long-term investments
Fair value	1.7	175.3
Common shares
Equity securities and Other long-term investments
Fair value	161.5	228.1
Other equity securities
Equity securities and Other long-term investments
Fair value	$ (13.3)	$ 1.8