Statutory Financial Information (Narrative) (Details) $ in Millions	Dec. 31, 2018 USD ($)
Statutory Accounting Practices [Line Items]
Amount of cash and investments held as funds at Lloyd's	$ 864.3
Funds at Lloyd's, Holding Company	311.6
United States [Member]
Statutory Accounting Practices [Line Items]
Amount available for dividend payment	466.0
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Amount available for dividend payment	$ 373.9