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
	Mar. 31, 2020	Mar. 31, 2019
Interest rate cap | Other revenues
Derivatives
Gains (losses) recognized in earnings	$ 0.0	$ (0.1)
Foreign currency swaps | Net foreign exchange gains (losses)
Derivatives
Gains (losses) recognized in earnings	6.0	0.8
Foreign currency forwards | Net foreign exchange gains (losses)
Derivatives
Gains (losses) recognized in earnings	0.5	0.2
Weather derivatives | Other revenues
Derivatives
Gains (losses) recognized in earnings	(20.4)	(6.0)
Equity futures contracts | Net realized investment gains (losses)
Derivatives
Gains (losses) recognized in earnings	2.9	(0.6)
Equity futures contracts | Net unrealized investment gains
Derivatives
Gains (losses) recognized in earnings	(1.0)	(0.2)
Equity put options | Net realized investment gains (losses)
Derivatives
Gains (losses) recognized in earnings	5.5	0.0
Equity put options | Net unrealized investment gains
Derivatives
Gains (losses) recognized in earnings	0.5	(0.4)
Equity warrants | Net unrealized investment gains
Derivatives
Gains (losses) recognized in earnings	$ 0.5	$ 0.0