Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Unrealized gains and losses on securities:
Net realized investment (gains) losses	$ (17.0)	$ 0.1	$ (25.1)	$ (6.9)
Provision for income tax	7.9	1.7	11.8	4.6
Net of taxes			(13.3)	(2.3)
Unrealized Holding Gains on Securities
Unrealized gains and losses on securities:
Net of taxes			(13.3)	(2.3)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Holding Gains on Securities
Unrealized gains and losses on securities:
Net realized investment (gains) losses	(17.0)	0.1	(25.1)	(6.9)
Provision for income tax	7.9	1.7	11.8	4.6
Net of taxes	$ (9.1)	$ 1.8	$ (13.3)	$ (2.3)