Common shareholder's equity, mezzanine equity, and non controlling interests - Non-controlling interests (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Change in non-controlling interest
Balance at beginning	$ 1.7	$ 0.2
Net income attributable to non-controlling interests	0.4	0.2
Other, net	0.1	0.1
Balance at end	2.2	$ 0.5
Alstead Re
Change in non-controlling interest
Balance at beginning	1.7
Balance at end	$ 2.1