Derivatives - Classification and fair value of derivatives in consolidated balance sheets (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivatives
Derivative assets	$ 11.7	$ 11.4
Derivative liabilities	20.4	9.5
Interest rate cap | Derivatives not designated as hedging instruments
Derivatives
Notional Value	250.0	250.0
Interest rate cap | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	0.0	0.0
Interest rate cap | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	0.0	0.0
Foreign currency swaps | Derivatives not designated as hedging instruments
Derivatives
Notional Value	90.0	90.0
Foreign currency swaps | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	2.4	0.0
Foreign currency swaps | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	0.0	3.6
Foreign currency forwards | Derivatives not designated as hedging instruments
Derivatives
Notional Value	(215.2)	(30.0)
Foreign currency forwards | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	7.3	2.7
Foreign currency forwards | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	5.7	5.7
Weather derivatives | Derivatives not designated as hedging instruments
Derivatives
Notional Value	105.9	110.7
Weather derivatives | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	0.0	7.0
Weather derivatives | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	13.9	0.0
Equity futures contracts | Derivatives not designated as hedging instruments
Derivatives
Notional Value	20.1	34.5
Equity futures contracts | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	0.0	0.0
Equity futures contracts | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	0.0	0.0
Equity call options | Derivatives not designated as hedging instruments
Derivatives
Notional Value	1.9	0.0
Equity call options | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	0.1	0.0
Equity call options | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	0.0	0.0
Equity put options | Derivatives not designated as hedging instruments
Derivatives
Notional Value	11.4	31.0
Equity put options | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	1.0	1.3
Equity put options | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	0.8	0.2
Equity warrants | Derivatives not designated as hedging instruments
Derivatives
Notional Value	0.9	0.4
Equity warrants | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	0.9	0.4
Equity warrants | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	$ 0.0	$ 0.0