Investment securities - Hedge funds and Private equity funds (Details) - Other Long-term Investments $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) fund	Dec. 31, 2019 USD ($)
Equity securities and Other long-term investments
Fair value	$ 388.0	$ 346.8
Long/short multi-sector
Equity securities and Other long-term investments
Fair value	54.9	53.0
Unfunded commitments	0.0	0.0
Distressed mortgage credit
Equity securities and Other long-term investments
Fair value	66.3	51.6
Unfunded commitments	0.0	0.0
Private credit
Equity securities and Other long-term investments
Fair value	22.1	21.5
Unfunded commitments	0.0	0.0
Multi-sector
Equity securities and Other long-term investments
Fair value	5.8	0.0
Unfunded commitments	0.0	0.0
Other
Equity securities and Other long-term investments
Fair value	1.2	1.4
Unfunded commitments	$ 0.0	0.0
Hedge funds
Equity securities and Other long-term investments
Number of investment funds in which the investments are held | fund	10
Fair value	$ 150.3	127.5
Unfunded commitments	0.0	0.0
Energy infrastructure & services
Equity securities and Other long-term investments
Fair value	56.6	53.6
Unfunded commitments	30.7	34.6
Multi-sector
Equity securities and Other long-term investments
Fair value	13.4	8.7
Unfunded commitments	8.1	7.8
Healthcare
Equity securities and Other long-term investments
Fair value	29.1	25.9
Unfunded commitments	6.3	10.4
Life settlement
Equity securities and Other long-term investments
Fair value	20.6	23.9
Unfunded commitments	0.0	0.0
Manufacturing/Industrial
Equity securities and Other long-term investments
Fair value	27.7	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	2.8	1.2
Unfunded commitments	$ 1.8	2.6
Private equity funds
Equity securities and Other long-term investments
Number of investment funds in which the investments are held | fund	32
Fair value	$ 150.8	141.5
Unfunded commitments	47.7	60.1
Hedge funds and private equity funds
Equity securities and Other long-term investments
Fair value	301.1	269.0
Unfunded commitments	$ 47.7	$ 60.1