Investment securities - Hedge Funds redemption frequency and Private Equity Fund lock-up period (Details) - Other long-term investments - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Equity securities and Other long-term investments
Other long-term investments	$ 389.7	$ 365.0
Hedge funds
Equity securities and Other long-term investments
Other long-term investments	123.1	118.3
Outstanding distribution from investments	0.0
Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	32.7
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	88.8
Private equity securities
Equity securities and Other long-term investments
Other long-term investments	178.6	183.1
Outstanding distribution from investments	53.2	$ 82.0
Private equity securities | Lock up period of 1 - 3 years
Equity securities and Other long-term investments
Other long-term investments	12.6
Private equity securities | Lock up period of 3 - 5 years
Equity securities and Other long-term investments
Other long-term investments	4.3
Private equity securities | Lock up period of 5 - 10 years
Equity securities and Other long-term investments
Other long-term investments	161.7
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	50.4
60-89 days notice | Hedge funds | Monthly Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	32.7
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	16.9
90-119 days notice | Hedge funds
Equity securities and Other long-term investments
Other long-term investments	51.6
90-119 days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	51.6
120+ days notice | Hedge funds
Equity securities and Other long-term investments
Other long-term investments	20.3
120+ days notice | Hedge funds | Annual Redemption Frequency
Equity securities and Other long-term investments
Other long-term investments	$ 20.3