Significant Accounting Policies Significant Accounting Policies ASU Impact (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income	$ (425.4)	$ 205.6	$ 229.8	$ 225.4	$ 225.8	$ 220.8	$ 209.7	$ 229.8	$ 235.4	$ 886.1	$ 852.6
Earnings Per Share, Diluted	$ (1.45)	$ 0.69	$ 0.75	$ 0.72	$ 0.71	$ 0.68	$ 0.63	$ 0.69	$ 0.78	$ 2.71	$ 2.57
Impairment of Deferred Acquisition Costs									289.8	0	0
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income									284.2	878.7	847.3
Earnings Per Share, Diluted									$ 0.94	$ 2.69	$ 2.55
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income									(12.1)	(7.4)	(5.3)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Diluted Earnings Per Share									$ (0.04)	$ (0.02)	$ (0.02)
Impact of Restatement on Earnings Per Share, Diluted									$ 0.62	$ 0.00	$ 0.00
Impairment of Deferred Acquisition Costs									188.4	0	0
Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of Restatement on Earnings Per Share, Diluted									$ (0.42)	$ 0.00	$ 0.00
Impairment of Deferred Acquisition Costs									$ (127.5)	$ 0	$ 0