Investment securities - Hedge Funds redemption frequency and Private Equity Fund lock-up period (Details) - Other long-term investments - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Equity securities and Other long-term investments
Fair value	$ 368.1	$ 346.8
Hedge funds
Equity securities and Other long-term investments
Fair value	138.1	127.5
Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	28.8
Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	87.0
Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.2
Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	22.1
Private equity funds
Equity securities and Other long-term investments
Fair value	151.8	$ 141.5
Private equity funds | Lock up period of 1 - 3 years
Equity securities and Other long-term investments
Fair value	54.6
Private equity funds | Lock up period of 3 - 5 years
Equity securities and Other long-term investments
Fair value	1.8
Private equity funds | Lock up period of 5 - 10 years
Equity securities and Other long-term investments
Fair value	95.4
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	51.2
60-89 days notice | Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	28.8
60-89 days notice | Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	22.2
60-89 days notice | Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.2
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	64.4
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	64.2
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	0.2
120+ days notice | Hedge funds
Equity securities and Other long-term investments
Fair value	21.9
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	$ 21.9