Investment securities - Hedge Funds redemption frequency and Private Equity Fund lock-up period (Details) - Other long-term investments - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Equity securities and Other long-term investments
Other long-term investments	$ 365.0	$ 269.5
Hedge funds
Equity securities and Other long-term investments
Other long-term investments	118.3	60.2
Outstanding distribution from investments	0.0
Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	25.4
Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	0.8
Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	0.8
Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	91.3
Private equity securities
Equity securities and Other long-term investments
Other long-term investments	183.1	145.1
Outstanding distribution from investments	82.0	$ 92.1
Private equity securities | Lock up period of 1 - 3 years
Equity securities and Other long-term investments
Other long-term investments	17.2
Private equity securities | Lock up period of 3 - 5 years
Equity securities and Other long-term investments
Other long-term investments	4.6
Private equity securities | Lock up period of 5 - 10 years
Equity securities and Other long-term investments
Other long-term investments	161.3
30-59 days notice | Hedge funds
Equity securities and Other long-term investments
Other long-term investments	0.8
30-59 days notice | Hedge funds | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	0.8
60-89 days notice | Hedge funds
Equity securities and Other long-term investments
Other long-term investments	41.8
60-89 days notice | Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	25.4
60-89 days notice | Hedge funds | Semi-annual Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	0.8
60-89 days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	15.6
90-119 days notice | Hedge funds
Equity securities and Other long-term investments
Other long-term investments	55.7
90-119 days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	55.7
120+ days notice | Hedge funds
Equity securities and Other long-term investments
Other long-term investments	20.0
120+ days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	$ 20.0