Acquisition-related charges (Details) (National [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		15 Months Ended	3 Months Ended								18 Months Ended	3 Months Ended								12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012 positions	Mar. 31, 2013 Minimum [Member]	Mar. 31, 2013 Maximum [Member]	Mar. 31, 2013 Amortization of intangible assets [Member]	Mar. 31, 2012 Amortization of intangible assets [Member]	Mar. 31, 2013 Retention bonuses [Member]	Mar. 31, 2012 Retention bonuses [Member]	Mar. 31, 2013 Severance and other benefits [Member]	Mar. 31, 2012 Severance and other benefits [Member]	Mar. 31, 2013 Severance and other benefits [Member]	Mar. 31, 2013 Stock-based compensation [Member]	Mar. 31, 2012 Stock-based compensation [Member]	Mar. 31, 2013 Transaction and other costs [Member]	Mar. 31, 2012 Transaction and other costs [Member]	Mar. 31, 2013 As recorded in Acquisition charges [Member]	Mar. 31, 2012 As recorded in Acquisition charges [Member]	Mar. 31, 2013 Distributor contract termination [Member]	Mar. 31, 2012 Distributor contract termination [Member]	Dec. 31, 2012 Distributor contract termination [Member]	Dec. 31, 2011 Distributor contract termination [Member] Distributors
Business Acquisition [Line Items]
Acquisition-related costs	$ 86	$ 174				$ 81	$ 81	$ 2	$ 41	$ 0	$ 12	$ 86	$ 3	$ 6	$ 0	$ 13	$ 86	$ 153	$ 0	$ 21	$ 21
Identified intangible assets subject to amortization, Weighted Average Useful Life (Years)				2 years	10 years
Restructuring and Related Cost, Number of Positions Eliminated			350
Business Acquisition, Amount Related to Severance and Other Benefit Costs Paid												$ 81
Business Acquistion, Number of distributors discontinued																						1