Unrealized Gains and Losses on Available-For-Sale and Other Securities (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Beginning balance	$ 1.9	$ 1.8
Net appreciation (depreciation) on available-for-sale securities	(151.3)	(65.1)	(97.8)
Portion of OTTI losses recognized in other comprehensive income	(12.2)	(10.5)	(6.9)
Ending balance	2.1	1.9	1.8
Accumulated Net Unrealized Investment Gain (Loss)
Schedule of Available-for-sale Securities [Line Items]
Beginning balance	308.7	218.3	107.7
Net appreciation (depreciation) on available-for-sale securities	151.3	65.1	97.8
Portion of OTTI losses recognized in other comprehensive income	12.2	10.5	6.9
Benefit for deferred income taxes	(46.2)	14.8	5.9
Total adjustment	117.3	90.4	110.6
Ending balance	426.0	308.7	218.3
Accumulated Net Unrealized Investment Gain (Loss) | Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Beginning balance	300.2	210.3	97.8
Net appreciation (depreciation) on available-for-sale securities	140.7	66.4	101.1
Portion of OTTI losses recognized in other comprehensive income	12.2	10.5	6.9
Benefit for deferred income taxes	(43.0)	13.0	4.5
Total adjustment	109.9	89.9	112.5
Ending balance	410.1	300.2	210.3
Accumulated Net Unrealized Investment Gain (Loss) | Equity Securities And Other
Schedule of Available-for-sale Securities [Line Items]
Beginning balance	8.5	8.0	9.9
Net appreciation (depreciation) on available-for-sale securities	10.6	(1.3)	(3.3)
Benefit for deferred income taxes	(3.2)	1.8	1.4
Total adjustment	7.4	0.5	(1.9)
Ending balance	$ 15.9	$ 8.5	$ 8.0