Variable interest entities - Unconsolidated VIE (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Total VIE Assets
Total VIE Assets	$ 388.0	$ 346.8
Total assets	6,632.8	6,413.8
Unconsolidated VIE
Total VIE Assets
Total VIE Assets	254.8	257.8
Total assets	254.8	257.8
On-Balance Sheet | Unconsolidated VIE
Total VIE Assets
Maximum Exposure to Loss	117.1	102.6
Total assets	117.1	102.6
Off-Balance Sheet | Unconsolidated VIE
Total VIE Assets
Maximum Exposure to Loss	7.4	16.3
Total assets	7.4	16.3
Total | Unconsolidated VIE
Total VIE Assets
Maximum Exposure to Loss	124.5	118.9
Total assets	$ 124.5	$ 118.9