Variable interest entities - Unconsolidated VIE (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Variable interest entities
Other long-term investments	$ 389.7	$ 365.0
Total assets	6,307.0	6,007.7
Unconsolidated VIE
Variable interest entities
Other long-term investments	220.6	209.1
Total assets	220.6	209.1
Other long-term investments
Variable interest entities
Other long term investments	143.8	135.1
Total assets	143.8	135.1
On-Balance Sheet
Variable interest entities
Other long term investments	118.5	103.1
Total assets	118.5	103.1
Off-Balance Sheet
Variable interest entities
Other long term investments	25.3	32.0
Total assets	$ 25.3	$ 32.0