Stockholders' Equity - Reconciliations of Statutory Capital and Surplus and Net Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stockholders' Equity Note [Abstract]
Statutory capital and surplus of insurance subsidiaries	$ 814.3				$ 778.4				$ 814.3	$ 778.4
Net liabilities of non-insurance parent and affiliates	(80.2)				(80.8)				(80.2)	(80.8)
Total	734.1				697.6				734.1	697.6
Increases (decreases):
Deferred acquisition costs	129.1				126.5				129.1	126.5	$ 96.8	$ 91.7
Postretirement and pension benefits	23.2				27.2				23.2	27.2
Deferred federal income taxes	(41.6)				(49.7)				(41.6)	(49.7)
Fixed maturities, at fair value	27.9				60.6				27.9	60.6
Other, net	11.9				10.7				11.9	10.7
Stockholders’ equity per accompanying consolidated financial statements	884.6				872.9				884.6	872.9	785.0
Statutory net (loss) income of insurance subsidiaries									65.4	(17.5)	51.1
Net income (loss) of non-insurance parent and affiliates									(4.1)	(5.3)	(5.1)
Total									(61.3)	22.8	(46.0)
Deferred acquisition costs									2.5	29.7	5.1
Postretirement and pension benefits									4.2	10.6	10.3
Deferred federal income taxes									(12.2)	79.3	(1.0)
Share-based compensation expense									(0.1)	(0.5)	(1.3)
Other, net									(4.5)	11.1	1.7
Net income	$ 3.1	$ 20.7	$ 2.7	$ 24.7	$ 65.4	$ 11.9	$ 3.0	$ 27.1	$ 51.2	$ 107.4	$ 60.8