Derivatives - Classification and fair value of derivatives in consolidated balance sheets (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivatives
Derivative assets	$ 7.7	$ 11.4
Derivative liabilities	3.6	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	40.0	90.0
Foreign currency swaps | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	0.3	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	225.2	30.0
Foreign currency forwards | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	0.3	2.7
Foreign currency forwards | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	0.0	5.7
Weather derivatives | Derivatives not designated as hedging instruments
Derivatives
Notional Value	53.6	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	3.5	0.0
Equity futures contracts | Derivatives not designated as hedging instruments
Derivatives
Notional Value	1.0	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
Foreign currency futures contracts | Derivatives not designated as hedging instruments
Derivatives
Notional Value	22.3	0.0
Foreign currency futures contracts | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	0.0	0.0
Foreign currency 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	34.6	0.0
Equity call options | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	2.5	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	77.3	31.0
Equity put options | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	1.9	1.3
Equity put options | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	0.1	0.2
Foreign currency call options | Derivatives not designated as hedging instruments
Derivatives
Notional Value	50.6	0.0
Foreign currency call options | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	2.2	0.0
Foreign currency call options | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	0.0	0.0
Equity warrants | Derivatives not designated as hedging instruments
Derivatives
Notional Value	0.5	0.4
Equity warrants | Derivatives not designated as hedging instruments | Other assets
Derivatives
Derivative assets	0.5	0.4
Equity warrants | Derivatives not designated as hedging instruments | Other liabilities
Derivatives
Derivative liabilities	$ 0.0	$ 0.0