Goodwill and intangible assets (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Amortization charges associated with finite-lived intangible assets	$ 397	$ 380	$ 423
Total estimated amortization of finite- lived intangible assets for 2013	464
Total estimated amortization of finite- lived intangible assets for 2014	446
Total estimated amortization of finite- lived intangible assets for 2015	434
Total estimated amortization of finite-lived intangible assets for 2016	413
Total estimated amortization of finite-lived intangible assets for 2017	$ 271
Minimum [Member]
Goodwill [Line Items]
Finite-lived intangible asset, useful life	3 years
Maximum [Member]
Goodwill [Line Items]
Finite-lived intangible asset, useful life	15 years