Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income by Component, Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ (57.3)	$ 53.4	$ 36.7	$ 32.5
Other comprehensive income before reclassifications	(11.6)	7.2	(48.7)	42.7
Amounts reclassified from AOCI	0.6	(12.7)	0.5	(27.3)
Other comprehensive loss	(11.0)	(5.5)	(48.2)	15.4
Ending balance	(68.3)	47.9	(68.3)	47.9
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018					$ (56.8)
Amounts reclassified from AOCI	0.6	(12.7)	0.5	(27.3)
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(20.3)	83.0	66.0	62.8
Other comprehensive income before reclassifications	(11.6)	7.2	(48.7)	42.7
Amounts reclassified from AOCI	0.0	(13.2)	(1.3)	(28.5)
Other comprehensive loss	(11.6)	(6.0)	(50.0)	14.2
Ending balance	(31.9)	77.0	(31.9)	77.0
Unrealized Gains and Losses on Available-for-Sale Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018					(47.9)
Benefit Plan Items [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(37.0)	(29.6)	(29.3)	(30.3)
Other comprehensive income before reclassifications	0.0	0.0	0.0	0.0
Amounts reclassified from AOCI	0.6	0.5	1.8	1.2
Other comprehensive loss	0.6	0.5	1.8	1.2
Ending balance	$ (36.4)	$ (29.1)	$ (36.4)	$ (29.1)
Benefit Plan Items [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018					$ (8.9)