Variable interest entities - Unconsolidated VIE (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Variable interest entities
Other long-term investments	$ 379.9	$ 365.0
Total assets	6,419.2	6,007.7
Unconsolidated VIE
Variable interest entities
Other long-term investments	266.3	209.1
Total assets	266.3	209.1
Other long-term investments
Variable interest entities
Other long term investments	138.5	135.1
Total assets	138.5	135.1
On-Balance Sheet
Variable interest entities
Other long term investments	120.2	103.1
Total assets	120.2	103.1
Off-Balance Sheet
Variable interest entities
Other long term investments	18.3	32.0
Total assets	$ 18.3	$ 32.0