Investment securities - Hedge funds and Private equity funds (Details) - Other long-term investments $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) fund	Dec. 31, 2019 USD ($)
Equity securities and Other long-term investments
Fair value	$ 355.5	$ 346.8
Hedge funds and private equity funds
Equity securities and Other long-term investments
Fair value	279.6	269.0
Unfunded commitments	$ 51.3	60.1
Hedge funds:
Equity securities and Other long-term investments
Number of investment funds in which the investments are held | fund	9
Fair value	$ 131.8	127.5
Unfunded commitments	0.0	0.0
Long/short multi-sector
Equity securities and Other long-term investments
Fair value	49.5	53.0
Unfunded commitments	0.0	0.0
Distressed mortgage credit
Equity securities and Other long-term investments
Fair value	59.4	51.6
Unfunded commitments	0.0	0.0
Private credit
Equity securities and Other long-term investments
Fair value	21.8	21.5
Unfunded commitments	0.0	0.0
Other
Equity securities and Other long-term investments
Fair value	1.1	1.4
Unfunded commitments	$ 0.0	0.0
Private equity funds:
Equity securities and Other long-term investments
Number of investment funds in which the investments are held | fund	33
Fair value	$ 147.8	141.5
Unfunded commitments	51.3	60.1
Energy infrastructure & services
Equity securities and Other long-term investments
Fair value	51.5	53.6
Unfunded commitments	34.1	34.6
Multi-sector
Equity securities and Other long-term investments
Fair value	16.0	8.7
Unfunded commitments	7.8	7.8
Healthcare
Equity securities and Other long-term investments
Fair value	28.2	25.9
Unfunded commitments	6.8	10.4
Life settlement
Equity securities and Other long-term investments
Fair value	22.9	23.9
Unfunded commitments	0.0	0.0
Manufacturing/Industrial
Equity securities and Other long-term investments
Fair value	26.3	27.6
Unfunded commitments	0.0	3.9
Private equity secondaries
Equity securities and Other long-term investments
Fair value	0.6	0.6
Unfunded commitments	0.8	0.8
Other
Equity securities and Other long-term investments
Fair value	2.3	1.2
Unfunded commitments	$ 1.8	$ 2.6