Investment securities - Hedge Funds and Private Equity Funds (Details) - Other long-term investments $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Equity securities and Other long-term investments
Largest investment in single fund	$ 54.8	$ 31.4
Other long-term investments	365.0	269.5
Hedge funds and private equity funds
Equity securities and Other long-term investments
Other long-term investments	301.4	205.3
Unfunded Commitments	$ 82.0	92.1
Hedge funds
Equity securities and Other long-term investments
Number of investment funds in which the investments are held | item	9
Other long-term investments	$ 118.3	60.2
Unfunded Commitments	0.0
Long/short multi-sector
Equity securities and Other long-term investments
Other long-term investments	41.0	31.5
Distressed mortgage credit
Equity securities and Other long-term investments
Other long-term investments	54.8	25.5
Private credit
Equity securities and Other long-term investments
Other long-term investments	20.0
Other
Equity securities and Other long-term investments
Other long-term investments	$ 2.5	3.2
Private equity securities
Equity securities and Other long-term investments
Number of investment funds in which the investments are held | item	27
Other long-term investments	$ 183.1	145.1
Unfunded Commitments	82.0	92.1
Energy infrastructure & services
Equity securities and Other long-term investments
Other long-term investments	93.7	73.5
Unfunded Commitments	54.2	56.4
Multi-sector
Equity securities and Other long-term investments
Other long-term investments	9.0	9.5
Unfunded Commitments	0.7	1.0
Healthcare
Equity securities and Other long-term investments
Other long-term investments	31.7	23.4
Unfunded Commitments	15.6	28.6
Life settlement
Equity securities and Other long-term investments
Other long-term investments	23.7	21.5
Manufacturing/Industrial
Equity securities and Other long-term investments
Other long-term investments	23.6	15.9
Unfunded Commitments	10.4	5.1
Private equity secondaries
Equity securities and Other long-term investments
Other long-term investments	1.1	1.1
Unfunded Commitments	1.1	1.0
Real estate
Equity securities and Other long-term investments
Other long-term investments	$ 0.3	$ 0.2