Debt (details) - Schedule of Debt (USD $) In Millions, unless otherwise specified			12 Months Ended				0 Months Ended														0 Months Ended													0 Months Ended							12 Months Ended		0 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Commercial Paper [Member]	Dec. 31, 2010 Commercial Paper [Member]	Dec. 31, 2011 5.375% Senior notes due June 15, 2012 [Member]	Dec. 31, 2010 5.375% Senior notes due June 15, 2012 [Member]	Jun. 01, 2011 7.22% Real estate non-recourse debt due September 1, 2011 [Member]	Dec. 31, 2010 7.22% Real estate non-recourse debt due September 1, 2011 [Member]	Dec. 31, 2011 5.00% Senior notes due March 15, 2013 [Member]	Dec. 31, 2010 5.00% Senior notes due March 15, 2013 [Member]	Dec. 31, 2011 5.50% Senior notes due December 1, 2015 [Member]	Dec. 31, 2010 5.50% Senior notes due December 1, 2015 [Member]	Dec. 31, 2011 6.25% Senior notes due June 20, 2016 [Member]	Dec. 31, 2010 6.25% Senior notes due June 20, 2016 [Member]	Dec. 31, 2011 5.75% Senior notes due December 15, 2017 [Member]	Dec. 31, 2010 5.75% Senior notes due December 15, 2017 [Member]	Dec. 31, 2011 5.80% Senior notes due May 15, 2018 [Member]	Dec. 31, 2010 5.80% Senior notes due May 15, 2018 [Member]	Dec. 31, 2011 5.90% Senior notes due June 2, 2019 [Member]	Dec. 31, 2010 5.90% Senior notes due June 2, 2019 [Member]	Nov. 01, 2010 3.90% Senior notes due November 1, 2020 [Member]	Dec. 31, 2011 3.90% Senior notes due November 1, 2020 [Member]	Dec. 31, 2010 3.90% Senior notes due November 1, 2020 [Member]	Dec. 31, 2011 7.75% Senior notes due April 15, 2026 [Member]	Dec. 31, 2010 7.75% Senior notes due April 15, 2026 [Member]	Dec. 31, 2011 7.625% Junior subordinated debentures due December 15, 2027 [Member]	Dec. 31, 2010 7.625% Junior subordinated debentures due December 15, 2027 [Member]	Dec. 31, 2011 6.375% Senior notes due March 15, 2033 [Member]	Dec. 31, 2010 6.375% Senior notes due March 15, 2033 [Member]	Dec. 31, 2011 6.75% Senior notes due June 20, 2036 [Member]	Dec. 31, 2010 6.75% Senior notes due June 20, 2036 [Member]	Dec. 31, 2011 6.25% Senior notes due June 15, 2037 [Member]	Dec. 31, 2010 6.25% Senior notes due June 15, 2037 [Member]	Nov. 01, 2010 5.35% Senior notes due November 1, 2040	Dec. 31, 2011 5.35% Senior notes due November 1, 2040	Dec. 31, 2010 5.35% Senior notes due November 1, 2040	Dec. 31, 2011 8.50% Junior subordinated debentures due December 15, 2045 [Member]	Dec. 31, 2010 8.50% Junior subordinated debentures due December 15, 2045 [Member]	Dec. 31, 2011 8.312% Junior subordinated debentures due July 1, 2046 [Member]	Dec. 31, 2010 8.312% Junior subordinated debentures due July 1, 2046 [Member]	Dec. 31, 2011 6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member]	Dec. 31, 2010 6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member]	Apr. 15, 2010 8.125% Senior notes due April 15, 2010 [Member]	Aug. 23, 2010 7.415% Medium-Term notes due August 23, 2010 [Member]	Sep. 16, 2010 7.81% Private placement notes due September 16, 2010 [Member]
Schedule of debt
Total short-term debt	$ 350	$ 109	$ 100	$ 100	$ 250			$ 9
Total long-term debt	6,269	6,519				250			500	500	400	400	400	400	450	450	500	500	500	500		500	500	200	200	125	125	500	500	400	400	800	800		750	750	56	56	73	73	115	115
Interest rate (percent)					5.375%			7.22%	5.00%		5.50%		6.25%		5.75%		5.80%		5.90%			3.90%		7.75%		7.625%		6.375%		6.75%		6.25%			5.35%		8.50%		8.312%		6.25%	6.25%	8.125%	7.415%	7.81%
Debt, effective interest rate (percent)																										6.147%											6.362%		6.362%
Unamortized fair value adjustment, debt	53	54																								18	19										16	16	19	19
Principal																					500													750								1,000
Net proceeds from issuance, senior notes																					496													738
Deferral of interest payments, description																																									The Company has the right, on one or more occasions, to defer the payment of interest on the debentures. The Company will not be required to settle deferred interest until it has deferred interest for five consecutive years or, if earlier, made a payment of current interest during a deferral period. The Company may defer interest for up to ten consecutive years without giving rise to an event of default. Deferred interest will accumulate additional interest at an annual rate equal to the annual interest rate then applicable to the debentures.
Debt, maturity, medium-term notes																																												21
Debt, maturity, senior notes																																											250		4
Debt, principal amount repaid							$ 9																																			$ 885
Debt, terms of redemption																					The senior notes are redeemable in whole at any time or in part from time to time, at the Company's option, at a redemption price equal to the greater of (a) 100% of the principal amount of senior notes to be redeemed or (b) the sum of the present values of the remaining scheduled payments of principal and interest on the senior notes to be redeemed (exclusive of interest accrued to the date of redemption) discounted to the date of redemption on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the then current Treasury rate (as defined) plus 15 basis points for the 2020 senior notes and 20 basis points for the 2040 notes.													The senior notes are redeemable in whole at any time or in part from time to time, at the Company's option, at a redemption price equal to the greater of (a) 100% of the principal amount of senior notes to be redeemed or (b) the sum of the present values of the remaining scheduled payments of principal and interest on the senior notes to be redeemed (exclusive of interest accrued to the date of redemption) discounted to the date of redemption on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the then current Treasury rate (as defined) plus 15 basis points for the 2020 senior notes and 20 basis points for the 2040 notes.							The Company can redeem the debentures at its option, in whole or in part, at any time on or after March 15, 2017 at a redemption price of 100% of the principal amount being redeemed plus accrued but unpaid interest. The Company can redeem the debentures at its option prior to March 15, 2017 (a) in whole at any time or in part from time to time or (b) in whole, but not in part, in the event of certain tax or rating agency events relating to the debentures, at a redemption price equal to the greater of 100% of the principal amount being redeemed and the applicable make-whole amount, in each case plus any accrued and unpaid interest. The debentures have a final maturity date of March 15, 2067 and a scheduled maturity date of March 15, 2037. The Company can redeem the debentures at its option any time (as described above) using any source of funds, including cash. If the Company chooses not to redeem the debentures, then during the 180-day period ending not more than 15 and not less than ten business days prior to the scheduled maturity date, the Company will be required to use commercially reasonable efforts to sell enough qualifying capital securities to permit repayment of the debentures at the scheduled maturity date. If any debentures remain outstanding after the scheduled maturity date, unless and until the Company redeems the debentures (as described above) using any source of funds, including cash, the Company shall be required to use its commercially reasonable efforts on a quarterly basis to raise sufficient proceeds from the sale of qualifying capital securities to permit the repayment in full of the debentures. If there are remaining debentures at the final maturity date, the Company is required to redeem the debentures using any source of funds. Qualifying capital securities are securities (other than common stock, qualifying warrants, mandatorily convertible preferred stock, debt exchangeable for common equity, and debt exchangeable for preferred equity) which generally are treated by the ratings agencies as having similar equity content to the debentures.
Debt, interest rate																																									From and including March 15, 2017, the debentures will bear interest at an annual rate equal to three-month LIBOR plus 2.215%, payable quarterly on March 15, June 15, September 15 and December 15 of each year.
Interest rate, minimum (percent)			0.10%	0.20%
Interest rate, maximum (percent)			0.30%	0.30%