Statutory Accounting Principles - Statutory Net Income (Loss) for Principal Operating Subsidiaries (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 7.1	$ 47.4	$ 55.1
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	(15.1)	(9.5)	(95.5)
United States [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 196.1	$ 110.8	$ 57.0