Dividend Restrictions and Statutory Financial Information - Subsidiary Statutory Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) syndicate	Dec. 31, 2018 USD ($)
Statutory Accounting Practices [Line Items]
Restricted net assets of consolidated subsidiaries	$ 3,500.0
Number of syndicates | syndicate	3
Bermuda
Statutory Accounting Practices [Line Items]
Minimum liquidity ratio percent (not less than)	75.00%
Maximum percentage reduction in total statutory capital without prior approval of Bermuda regulator (percent) (or more)	15.00%
Minimum solvency requirement	$ 1,900.0	$ 2,100.0
United Kingdom
Statutory Accounting Practices [Line Items]
Minimum capital requirements	695.6	60.7
United States
Statutory Accounting Practices [Line Items]
Minimum levels of risk-based capital	488.3	359.6
Liechtenstein
Statutory Accounting Practices [Line Items]
Minimum solvency requirement	$ 119.0	$ 133.9