Stock-based compensation (National Acquisition-Related Equity Awards) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Sep. 23, 2011
Business Combinations [Abstract]
Future compensation cost not yet recognized in the statements of income	$ 477
National [Member]
Business Combinations [Abstract]
Acquisition-related equity awards, Total estimated fair value		147
Acquisition-related equity awards, Pre-combination vested service provided by employees, included in the total consideration transferred		22
Acquisition-related equity awards, Post-combination stock-based compensation expense recognized as employees provide service over the remaining vesting periods		125
Future compensation cost not yet recognized in the statements of income	$ 68

As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph h

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant