Variable interest entities - Unconsolidated VIE (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Variable interest entities
Other long-term investments	$ 368.1	$ 346.8
Total assets	6,664.0	6,413.8
Unconsolidated VIE
Variable interest entities
Other long-term investments	261.1	257.8
Total assets	261.1	257.8
On-Balance Sheet
Variable interest entities
Other long term investments	107.8	102.6
Total assets	107.8	102.6
Off-Balance Sheet
Variable interest entities
Other long term investments	8.3	16.3
Total assets	8.3	16.3
Other long-term investments
Variable interest entities
Other long term investments	116.1	118.9
Total assets	$ 116.1	$ 118.9