Derivatives - Classification and fair value of derivatives in consolidated of (loss) income statements (Details) - Derivatives not designated as hedging instruments - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Interest rate cap | Other revenues
Derivatives
Gains (losses) recognized in earnings	$ 0.0	$ 0.0	$ 0.0	$ (0.2)
Foreign currency swaps | Net foreign exchange (losses) gains
Derivatives
Gains (losses) recognized in earnings	(0.9)	3.7	1.8	6.1
Foreign currency forwards | Net foreign exchange (losses) gains
Derivatives
Gains (losses) recognized in earnings	(0.4)	(7.8)	(0.9)	(10.8)
Weather derivatives | Other revenues
Derivatives
Gains (losses) recognized in earnings	(0.4)	1.9	(21.8)	(3.6)
Equity futures contracts | Net realized investment (losses) gains
Derivatives
Gains (losses) recognized in earnings	(0.2)	0.1	2.4	(0.7)
Equity futures contracts | Net unrealized investment gains (losses)
Derivatives
Gains (losses) recognized in earnings	(0.1)	(0.5)	0.4	(0.6)
Foreign currency futures contracts | Net foreign exchange (losses) gains
Derivatives
Gains (losses) recognized in earnings	0.8	0.0	0.1	0.0
Equity put options | Net realized investment (losses) gains
Derivatives
Gains (losses) recognized in earnings	1.9	(0.8)	7.8	(0.8)
Equity put options | Net unrealized investment gains (losses)
Derivatives
Gains (losses) recognized in earnings	(0.6)	0.7	0.8	0.2
Foreign currency call options | Net foreign exchange (losses) gains
Derivatives
Gains (losses) recognized in earnings	1.0	0.0	0.8	0.0
Equity warrants | Net unrealized investment gains (losses)
Derivatives
Gains (losses) recognized in earnings	$ (0.1)	$ 0.3	$ 0.1	$ 0.7