Debt (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2008
Debt Instrument [Line Items]
Percentage of senior notes principal amount which may be redeemed at our option	100.00%
Cumulative adjustment to carrying value of senior note			$ 103,000,000
Weighted-average effective interest rate fixed			6.08%
Unamortized carrying value adjustment	74,000,000	84,000,000
Revolving credit agreement term, in years	3
Amount of unsecured revolving credit agreement	1,000,000,000
Expiration date of credit agreement	December 2013
Minimum level of net worth required	6,000,000,000
Actual net wroth	8,100,000,000
Maximum leverage ratios for new credit agreement	3.0
Actual leverage ratio	0.6
Number of bais point in addition to LIBOR	310
Outstanding letters of credit	14,000,000
Borrowings outstanding under credit agreement	0
Remaining borrowing capacity	986,000,000
Line of credit facility, covenant terms	The terms of the new credit agreement include standard provisions related to conditions of borrowing, including a customary material adverse effect clause which could limit our ability to borrow additional funds. In addition, the new credit agreement contains customary restrictive and financial covenants as well as customary events of default, including financial covenants regarding the maintenance of a minimum level of net worth of $6.0 billion at December 31, 2011 and a maximum leverage ratio of 3.0:1. We are in compliance with the financial covenants, with actual net worth of $8.1 billion and actual leverage ratio of 0.6:1, as measured in accordance with the new credit agreement as of December 31, 2011.
Uncommitment incremental loan facility	250,000,000
Portion of other long-term borrowings	36,000,000
New Credit Agreement [Member]
Debt Instrument [Line Items]
Revolving credit agreement term, in years	5
Amount of unsecured revolving credit agreement	$ 1,000,000,000
Expiration date of credit agreement	November 2016
Current basis points spread over LIBOR of revolving credit agreement	120
Lower limit of spread variance that is dependent upon credit rating, basis points	87.5
Upper limit of spread variance that is dependent upon credit rating, basis points	147.5
Current facility fee, basis points	17.5
Lower limit of facility fee that is dependent upon credit rating, basis points	12.5
Upper limit of facility fee that is dependent upon credit rating, basis points	27.5
Junior Subordinated Debt From Acquisition [Member]
Debt Instrument [Line Items]
Junior subordinated debt, interest rate beginning 2012	LIBOR plus 310 basis points
Junior Subordinated Debt From Acquisition [Member] | Other Long Term Borrowings [Member]
Debt Instrument [Line Items]
Stated interest rate	8.02%
Maturity date	Dec. 31, 2037
Junior subordinated debt callable date	Dec. 31, 2012
Payment terms	quarterly
$500 Million, 7.20% Due June 15, 2018 [Member]
Debt Instrument [Line Items]
Stated interest rate	7.20%
Maturity date	Jun. 15, 2018
$250 Million, 8.15% Due June 15, 2038 [Member]
Debt Instrument [Line Items]
Stated interest rate	8.15%
Maturity date	Jun. 15, 2038