Schedule V - Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Valuation And Qualifying Accounts [Abstract]
Valuation allowance for deferred tax asset, Balance at Beginning of Year	$ 22.8	$ 25.4	$ 56.2
Valuation allowance for deferred tax asset, Charged to Cost and Expense	(1.0)	(1.0)	(30.7)
Valuation allowance for deferred tax asset, Capital Loss Carryforward	0.0	0.0	0.0
Valuation allowance for deferred tax asset, Net Operating Loss Carryforward	0.0	0.0	0.0
Valuation allowance for deferred tax asset, Charged to Other Accounts	1.7	(1.6)	(0.1)
Valuation allowance for deferred tax asset, Deductions	0.0	0.0	0.0
Valuation allowance for deferred tax asset, Balance at End of Year	$ 23.5	$ 22.8	$ 25.4