Significant Accounting Policies ASU Impact (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferral of Acquisition Costs									$ (467.3)	$ (442.5)	$ (422.5)
Amortization of Deferred Acquisition Costs									378.7	365.7	373.3
Impairment of Deferred Acquisition Costs									0	196.0	0
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	873.5				614.8				873.5	614.8
Net Income	233.9	230.2	216.4	213.9	(369.0)	202.0	227.6	223.6	894.4	284.2	878.7
Increase (Decrease) in Deferred Policy Acquisition Costs									88.6	76.8	49.2
Earnings Per Share, Basic	$ 0.86	$ 0.83	$ 0.76	$ 0.74	$ (1.26)	$ 0.68	$ 0.74	$ 0.72	$ 3.18	$ 0.94	$ 2.70
Earnings Per Share, Diluted	$ 0.85	$ 0.83	$ 0.76	$ 0.73	$ (1.26)	$ 0.68	$ 0.74	$ 0.71	$ 3.17	$ 0.94	$ 2.69
Income Tax (Benefit) - Deferred									148.5	(181.4)	140.2
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	(72.6)				(117.6)				(72.6)	(117.6)
Change in Adjustment to Reserves for Future Policy and Contract Benefits Net of Reinsurance and Other net of tax expense benefit									(660.1)	(1,321.1)	(948.3)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax									45.0	(10.5)	(31.8)
Stockholders' Equity Attributable to Parent	8,612.6				8,169.7				8,612.6	8,169.7	8,484.9
Other Comprehensive Income (Loss), Net of Tax									166.2	110.4	3.9
Change in Income Taxes									168.0	(274.4)	160.3
Deferred Policy Acquisition Costs	1,755.5				1,677.1				1,755.5	1,677.1	1,816.1	1,784.3
Deferred Income Tax	269.4				44.7				269.4	44.7
Retained Earnings (Accumulated Deficit)	7,371.6				6,611.0				7,371.6	6,611.0
Previous Accounting Guidance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferral of Acquisition Costs										(628.3)	(607.7)
Amortization of Deferred Acquisition Costs										533.8	547.1
Impairment of Deferred Acquisition Costs										289.8	0
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax					605.8					605.8
Net Income										235.4	886.1
Increase (Decrease) in Deferred Policy Acquisition Costs										(94.5)	(60.6)
Earnings Per Share, Basic										$ 0.78	$ 2.72
Earnings Per Share, Diluted										$ 0.78	$ 2.71
Income Tax (Benefit) - Deferred										(208.7)	144.2
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax					(121.5)					(121.5)
Change in Adjustment to Reserves for Future Policy and Contract Benefits Net of Reinsurance and Other net of tax expense benefit										(1,324.4)	(950.9)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax										(10.6)	(32.2)
Other Comprehensive Income (Loss), Net of Tax										107.0	0.9
Change in Income Taxes										(301.7)	164.3
Deferred Policy Acquisition Costs					2,300.9					2,300.9
Deferred Income Tax					261.2					261.2
Retained Earnings (Accumulated Deficit)					7,031.2					7,031.2
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferral of Acquisition Costs										(442.5)	(422.5)
Amortization of Deferred Acquisition Costs										365.7	373.3
Impairment of Deferred Acquisition Costs										196.0	0
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax					614.8					614.8
Net Income										284.2	878.7
Increase (Decrease) in Deferred Policy Acquisition Costs										(76.8)	(49.2)
Earnings Per Share, Basic										$ 0.94	$ 2.70
Earnings Per Share, Diluted										$ 0.94	$ 2.69
Income Tax (Benefit) - Deferred										(181.4)	140.2
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax					(117.6)					(117.6)
Change in Adjustment to Reserves for Future Policy and Contract Benefits Net of Reinsurance and Other net of tax expense benefit										(1,321.1)	(948.3)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax										(10.5)	(31.8)
Other Comprehensive Income (Loss), Net of Tax										110.4	3.9
Change in Income Taxes										(274.4)	160.3
Deferred Policy Acquisition Costs					1,677.1					1,677.1
Deferred Income Tax					44.7					44.7
Retained Earnings (Accumulated Deficit)					6,611.0					6,611.0
Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferral of Acquisition Costs										185.8	185.2
Amortization of Deferred Acquisition Costs										(168.1)	(173.8)
Impairment of Deferred Acquisition Costs										(93.8)	0
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax					9.0					9.0
Net Income										48.8	(7.4)
Increase (Decrease) in Deferred Policy Acquisition Costs										17.7	11.4
Earnings Per Share, Basic										$ 0.16	$ (0.02)
Earnings Per Share, Diluted										$ 0.16	$ (0.02)
Income Tax (Benefit) - Deferred										27.3	(4.0)
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax					3.9					3.9
Change in Adjustment to Reserves for Future Policy and Contract Benefits Net of Reinsurance and Other net of tax expense benefit										3.3	2.6
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax										0.1	0.4
Other Comprehensive Income (Loss), Net of Tax										3.4	3.0
Change in Income Taxes										27.3	(4.0)
Deferred Policy Acquisition Costs					(623.8)					(623.8)
Deferred Income Tax					(216.5)					(216.5)
Retained Earnings (Accumulated Deficit)					(420.2)					(420.2)
Accumulated Other Comprehensive Income (Loss) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent	628.0				461.8				628.0	461.8	351.4	347.5
Other Comprehensive Income (Loss), Net of Tax									166.2	110.4	3.9
Accumulated Other Comprehensive Income (Loss) [Member] | Previous Accounting Guidance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent											341.9	341.0
Accumulated Other Comprehensive Income (Loss) [Member] | Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent											351.4	347.5
Accumulated Other Comprehensive Income (Loss) [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent											9.5	6.5
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income									894.4	284.2	878.7
Stockholders' Equity Attributable to Parent	7,371.6				6,611.0				7,371.6	6,611.0	6,591.8	5,827.9
Retained Earnings [Member] | Previous Accounting Guidance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent											7,060.8	6,289.5
Retained Earnings [Member] | Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent											6,591.8	5,827.9
Retained Earnings [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent											(469.0)	(461.6)
Before Tax [Member] | Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impairment of Deferred Acquisition Costs					$ 196.0					$ 196.0	$ 0