Investment securities - Hedge Funds redemption frequency and Private Equity Fund lock-up period (Details) - Other long-term investments - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Equity securities and Other long-term investments
Fair value	$ 355.5	$ 346.8
Hedge funds:
Equity securities and Other long-term investments
Fair value	131.8	127.5
Hedge funds: | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	33.5
Hedge funds: | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.6
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	97.5
Private equity funds:
Equity securities and Other long-term investments
Fair value	147.8	$ 141.5
Private equity funds: | Lock up period of 1 - 3 years
Equity securities and Other long-term investments
Fair value	9.1
Private equity funds: | Lock up period of 3 - 5 years
Equity securities and Other long-term investments
Fair value	2.1
Private equity funds: | Lock up period of 5 - 10 years
Equity securities and Other long-term investments
Fair value	136.6
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	49.7
60-89 days notice | Hedge funds: | Monthly Redemption Frequency
Equity securities and Other long-term investments
Fair value	33.5
60-89 days notice | Hedge funds: | Quarterly Redemption Frequency
Equity securities and Other long-term investments
Fair value	0.0
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	16.0
90-119 days notice | Hedge funds:
Equity securities and Other long-term investments
Fair value	59.6
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	0.0
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	59.6
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