Summary of assets and liabilities recorded at fair value (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	$ 9,625		$ 1,949
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value of Deferred Compensation Plan, Assets	54	[1]	44	[1]
Derivatives, assets	0		0
Fair Value Assets, total	8,343		1,621
Contingent Consideration	0		0
Fair Value of Deferred Compensation Plan, Liabilities	54	[1]	44	[1]
Derivatives, liabilities	0		0
Fair Value Liabilities, total	54		44
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value of Deferred Compensation Plan, Assets	0	[1]	0	[1]
Derivatives, assets	349		14
Fair Value Assets, total	1,685		386
Contingent Consideration	0		0
Fair Value of Deferred Compensation Plan, Liabilities	0	[1]	0	[1]
Derivatives, liabilities	0		99
Fair Value Liabilities, total	0		99
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value of Deferred Compensation Plan, Assets	0	[1]	0	[1]
Derivatives, assets	0		0
Fair Value Assets, total	0		0
Contingent Consideration	133		264
Fair Value of Deferred Compensation Plan, Liabilities	0	[1]	0	[1]
Derivatives, liabilities	0		0
Fair Value Liabilities, total	133		264
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value of Deferred Compensation Plan, Assets	54	[1]	44	[1]
Derivatives, assets	349		14
Fair Value Assets, total	10,028		2,007
Contingent Consideration	133		264
Fair Value of Deferred Compensation Plan, Liabilities	54	[1]	44	[1]
Derivatives, liabilities	0		99
Fair Value Liabilities, total	187		407
Money market funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	7,926		1,491
Money market funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
Money market funds [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
Money market funds [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	7,926		1,491
Corporate debt securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
Corporate debt securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	938		220
Corporate debt securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
Corporate debt securities [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	938		220
US treasury securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	363		86
US treasury securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
US treasury securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
US treasury securities [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	363		86
Residential mortgage-backed and asset-backed securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
Residential mortgage-backed and asset-backed securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	269		47
Residential mortgage-backed and asset-backed securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
Residential mortgage-backed and asset-backed securities [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	269		47
US Government Agencies Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
US Government Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	113		93
US Government Agencies Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
US Government Agencies Securities [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	113		93
Municipal debt securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
Municipal debt securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	16		12
Municipal debt securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	0		0
Municipal debt securities [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value	$ 16		$ 12

[1] We maintain a deferred compensation plan under which our directors and key employees may defer compensation for income tax purposes. Amounts deferred by participants are deposited in a rabbit trust and the amount is periodically adjusted to reflect earnings or (losses) based on the participant's investment elections among a select group of investment funds which consists of money market funds and mutual funds.