Common shareholder's equity, mezzanine equity, and non controlling interests - Non-controlling interests (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Common shareholder's equity, mezzanine equity, and non-controlling interests
Non-controlling interests	$ 2.5	$ 3.4	$ 2.4	$ 1.7
Change in non-controlling interest
Non-controlling interests, beginning of the period	2.5	3.0	2.4	1.7
Net income attributable to non-controlling interests	0.2	0.4	0.2	1.6
Dividends paid	(1.0)	0.0	(1.0)	0.0
Other, net	0.1	0.0	0.2	0.1
Non-controlling interests, end of the period	1.8	$ 3.4	1.8	$ 3.4
Alstead Re
Common shareholder's equity, mezzanine equity, and non-controlling interests
Non-controlling interests	1.8		2.3
Change in non-controlling interest
Non-controlling interests, beginning of the period			2.3
Non-controlling interests, end of the period	$ 1.8		$ 1.8