Common shareholder's equity, mezzanine equity, and non controlling interests - Non-controlling interests (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Common shareholder's equity, mezzanine equity, and non-controlling interests
Non-controlling interests	$ 2.5	$ 3.0	$ 2.5	$ 3.0
Change in non-controlling interest
Non-controlling interests, beginning of the period	2.6	2.2	2.4	1.7
Net income (loss) attributable to non-controlling interests	(0.2)	0.8	0.0	1.2
Other, net	0.1	0.0	0.1	0.1
Non-controlling interests, end of the period	2.5	$ 3.0	2.5	$ 3.0
Alstead Re
Common shareholder's equity, mezzanine equity, and non-controlling interests
Non-controlling interests	2.3		2.3
Change in non-controlling interest
Non-controlling interests, beginning of the period			2.3
Non-controlling interests, end of the period	$ 2.3		$ 2.3