Fair Value Measurements (Fair value of financial instruments) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Level 1 [Member] | Carrying Amount [Member]
Level 1:
Subordinated convertible debentures	$ 0		$ 55
Senior and senior subordinated notes	5,383		5,387
Level 1 [Member] | Fair Value [Member]
Level 1:
Subordinated convertible debentures	0		63
Senior and senior subordinated notes	5,773		5,881
Level 2 [Member] | Carrying Amount [Member]
Level 2:
4 percent Convertible Senior Notes	134	[1]	137	[1]
Level 2 [Member] | Fair Value [Member]
Level 2:
4 percent Convertible Senior Notes	$ 146	[1]	$ 155	[1]
4 percent Convertible Subordinated Notes [Member]
Level 2:
Stated interest rate	4.00%
Effective interest rate	6.80%

[1] The fair value of the 4 percent Convertible Senior Notes is based on the market value of comparable notes. Consistent with the carrying amount, the fair value excludes the equity component of the notes. To exclude the equity component and calculate the fair value, we used an effective interest rate of 6.8 percent.