Notes Payable and Debt and Financing Arrangements (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Jun. 21, 2007
Proceeds from Issuance of Senior Long-term Debt	$ 600,000,000
Debt Instrument, Interest Rate, Effective Percentage	6.50%
Required Statutory Capital And Surplus For Principal Operating Subsidiares		6,200,000,000
Equity Unit Note Coupon Rate	8.25%
Senior Notes	575,000,000
Rate of interest of Equity security Units	10.75%
Equity Security Unit Settlement Shares (in Shares)	1.3242
Face Value Of Equity Security Unit Issued (in Dollars per share)	$ 25
Senior Notes, Principal Amount	400,000,000
Line of Credit Facility, Maximum Borrowing Capacity	1,000,000,000
Letter of credit facility existing limit			4,000,000,000
Letter of credit facility revised limit	3,000,000,000
Relating To Settlement of Purchase of Contracts [Member] | Common Class A [Member]
Stock Issued During Period, Shares, New Issues (in Shares)	30,456,600
Senior Notes [Member]
Proceeds from Issuance of Senior Long-term Debt	$ 396,400,000
Debt Instrument, Interest Rate, Stated Percentage	5.75%