Investment securities - Hedge Funds redemption frequency and Private Equity Fund lock-up period (Details) - Other Long-term Investments - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Equity securities and Other long-term investments
Fair value	$ 388.0	$ 346.8
Hedge funds
Equity securities and Other long-term investments
Fair value	150.3	127.5
Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	36.3
Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	91.3
Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.3
Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	22.4
Private equity funds
Equity securities and Other long-term investments
Fair value	150.8	$ 141.5
Private equity funds | Lock up period of 1 - 3 years
Equity securities and Other long-term investments
Fair value	53.9
Private equity funds | Lock up period of 3 - 5 years
Equity securities and Other long-term investments
Fair value	19.0
Private equity funds | Lock up period of 5 - 10 years
Equity securities and Other long-term investments
Fair value	77.9
1-29 days notice | Hedge funds
Equity securities and Other long-term investments
Fair value	5.8
1-29 days notice | Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	5.8
1-29 days notice | Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
1-29 days notice | Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
1-29 days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
30-59 days notice | Hedge funds
Equity securities and Other long-term investments
Fair value	0.6
30-59 days notice | Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
30-59 days notice | Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.6
30-59 days notice | Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
30-59 days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
60-89 days notice | Hedge funds
Equity securities and Other long-term investments
Fair value	55.2
60-89 days notice | Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	30.5
60-89 days notice | Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	24.4
60-89 days notice | Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.3
60-89 days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
90-119 days notice | Hedge funds
Equity securities and Other long-term investments
Fair value	66.6
90-119 days notice | Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
90-119 days notice | Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	66.3
90-119 days notice | Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
90-119 days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.3
120+ days notice | Hedge funds
Equity securities and Other long-term investments
Fair value	22.1
120+ days notice | Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
120+ days notice | Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
120+ days notice | Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
120+ days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	$ 22.1