Investment securities - Equity securities and Other long-term investments (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Equity securities
Equity securities and Other long-term investments
Cost or amortized cost	$ 376.2	$ 409.4
Gross unrealized gains	38.6	17.8
Gross unrealized losses	(36.7)	(50.8)
Net foreign currency gains (losses)	9.7	3.6
Fair value	387.8	380.0
Other long-term investments
Equity securities and Other long-term investments
Cost or amortized cost	349.9	337.6
Gross unrealized gains	44.2	32.6
Gross unrealized losses	(24.6)	(13.5)
Net foreign currency gains (losses)	10.4	8.3
Fair value	379.9	365.0
Other long-term investments | Hedge funds and private equity funds
Equity securities and Other long-term investments
Fair value	291.9	301.4
Other long-term investments | Limited liability companies and private equity securities
Equity securities and Other long-term investments
Fair value	88.0	63.6
Investment grade fixed income mutual funds
Equity securities and Other long-term investments
Fair value	151.9	157.7
Common shares
Equity securities and Other long-term investments
Fair value	233.4	$ 222.3
Other
Equity securities and Other long-term investments
Fair value	$ 2.5