Accounting Developments - ASU Impact (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Acquisition Costs	$ 1,699.7		$ 1,677.1
Deferred Income Tax	88.9		44.7
Net Unrealized Gain on Securities Not Other-Than-Temporarily Impaired	584.4		614.8
Foreign Currency Translation Adjustment	(89.4)		(117.6)
Retained Earnings	6,793.5		6,611.0
Deferral of Acquisition Costs	(117.4)	(112.2)
Amortization of Deferred Acquisition Costs	99.4	95.0
Income Tax - Deferred	58.8	38.5
Net Income	213.9	223.6
Earnings Per Share, Basic	$ 0.74	$ 0.72
Earnings Per Share, Assuming Dilution	$ 0.73	$ 0.71
Change in Adjustment to Reserves for Future Policy and Contract Benefits, Net of Reinsurance and Other	166.8	113.2
Change in Foreign Currency Translation Adjustment	28.2	26.5
Change in Deferred Acquisition Costs	(18.0)	(17.2)
Change in Income Taxes	116.7	82.0
Previous Accounting Guidance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Acquisition Costs		2,542.9	2,300.9
Deferred Income Tax		471.2	261.2
Net Unrealized Gain on Securities Not Other-Than-Temporarily Impaired		443.6	605.8
Foreign Currency Translation Adjustment		(84.1)	(121.5)
Retained Earnings		7,112.3	7,031.2
Deferral of Acquisition Costs		(158.3)
Amortization of Deferred Acquisition Costs		138.3
Income Tax - Deferred		39.5
Net Income		225.4
Earnings Per Share, Basic		$ 0.72
Earnings Per Share, Assuming Dilution		$ 0.72
Change in Adjustment to Reserves for Future Policy and Contract Benefits, Net of Reinsurance and Other		113.2
Change in Foreign Currency Translation Adjustment		26.8
Change in Deferred Acquisition Costs		(20.0)
Change in Income Taxes		83.0
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Acquisition Costs		1,834.6	1,677.1
Deferred Income Tax		224.5	44.7
Net Unrealized Gain on Securities Not Other-Than-Temporarily Impaired		449.3	614.8
Foreign Currency Translation Adjustment		(80.6)	(117.6)
Retained Earnings		6,641.5	6,611.0
Deferral of Acquisition Costs		(112.2)
Amortization of Deferred Acquisition Costs		95.0
Income Tax - Deferred		38.5
Net Income		223.6
Earnings Per Share, Basic		$ 0.72
Earnings Per Share, Assuming Dilution		$ 0.71
Change in Adjustment to Reserves for Future Policy and Contract Benefits, Net of Reinsurance and Other		113.2
Change in Foreign Currency Translation Adjustment		26.5
Change in Deferred Acquisition Costs		(17.2)
Change in Income Taxes		82.0
Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Acquisition Costs		(708.3)	(623.8)
Deferred Income Tax		(246.7)	(216.5)
Net Unrealized Gain on Securities Not Other-Than-Temporarily Impaired		5.7
Foreign Currency Translation Adjustment		3.5	3.9
Retained Earnings		(470.8)	(420.2)
Deferral of Acquisition Costs		46.1
Amortization of Deferred Acquisition Costs		(43.3)
Income Tax - Deferred		(1.0)
Net Income		(1.8)
Earnings Per Share, Basic		$ 0.00
Earnings Per Share, Assuming Dilution		$ (0.01)
Change in Adjustment to Reserves for Future Policy and Contract Benefits, Net of Reinsurance and Other		0
Change in Foreign Currency Translation Adjustment		(0.3)
Change in Deferred Acquisition Costs		2.8
Change in Income Taxes		$ (1.0)