Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Settlement of forward starting swaps	$ (121)
Business acquisition assumed debt	120
Repayment of acquired debt	0	104
Maturity of 6 7/8% notes	600	71
Notes 5.75% due 2013 [Member]
Schedule of Held-to-maturity Securities [Line Items]
Maturity of 6 7/8% notes	600
Notes 6.875% [Member]
Schedule of Held-to-maturity Securities [Line Items]
Stated interest rate	6.875%
Dyrup [Member]
Schedule of Held-to-maturity Securities [Line Items]
Business acquisition assumed debt	$ 120