Reconciliation of Gross Unrealized Investment Gains or losses to Net Balance (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Investments Unrealized Gain Loss Positions [Line Items]
Unrealized gains arising during period, Pre-Tax	$ 90.9	$ 15.4	$ 191.3	$ 74.6
Less: reclassification adjustments for gains realized in net income, Pre-Tax	4.3	8.4	7.3	24.7
Total available-for-sale securities, Pre-Tax	86.6	7.0	184.0	49.9
Unrealized losses arising during period, Pre-Tax		0.1	0.1	(1.8)
Other comprehensive income (loss)	86.6	7.1	184.1	48.1
Unrealized gains arising during period, Tax Benefit (Expense)	(32.1)	7.8	(35.6)	12.9
Less: reclassification adjustments for gains realized in net income	2.7	(1.9)	7.9	2.4
Total available-for-sale securities, Tax Benefit (Expense)	(34.8)	9.7	(43.5)	10.5
Unrealized losses arising during period, Tax Benefit (Expense)		(0.1)		0.6
Other comprehensive income (loss)	(34.8)	9.6	(43.5)	11.1
Unrealized gains arising during period, Net of Tax	58.8	23.2	155.7	87.5
Less: reclassification adjustments for gains realized in net income, Net of Tax	7.0	6.5	15.2	27.1
Total available-for-sale securities	51.8	16.7	140.5	60.4
Unrealized losses arising during period, Net of Tax			0.1	(1.2)
Other comprehensive income (loss)	$ 51.8	$ 16.7	$ 140.6	$ 59.2