Financing arrangements (Debt Exchange) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Aggregate principal amount of notes issued	$ 6,700,000,000	$ 3,500,000,000.0	$ 4,500,000,000.0
Unamortized bond discounts and issuance costs	$ (936,000,000)	$ (210,000,000)
4.563% notes due 2048 (4.563% 2048 Notes) [Member]
Debt Instrument [Line Items]
Stated contractual interest rate on note	4.563%
4.663% notes due 2051 (4.663% 2051 Notes) [Member]
Debt Instrument [Line Items]
Stated contractual interest rate on note	4.663%
6.375% notes due 2037 (6.375% 2037 Notes) [Member]
Debt Instrument [Line Items]
Stated contractual interest rate on note	6.375%
6.90% notes due 2038 (6.90% 2038 Notes) [Member]
Debt Instrument [Line Items]
Stated contractual interest rate on note	6.90%
6.40% notes due 2039 (6.40% 2039 Notes) [Member]
Debt Instrument [Line Items]
Stated contractual interest rate on note	6.40%
5.75% notes due 2040 (5.75% 2040 Notes) [Member]
Debt Instrument [Line Items]
Stated contractual interest rate on note	5.75%
5.15% notes due 2041 (5.15% 2041 Notes) [Member]
Debt Instrument [Line Items]
Stated contractual interest rate on note	5.15%
5.65% notes due 2042 (5.65% 2042 Notes) [Member]
Debt Instrument [Line Items]
Stated contractual interest rate on note	5.65%
5.375% notes due 2043 (5.375% 2043 Notes) [Member]
Debt Instrument [Line Items]
Stated contractual interest rate on note	5.375%
Notes payable to banks [Member]
Debt Instrument [Line Items]
Unamortized bond discounts and issuance costs	$ 801,000,000
Notes payable to banks [Member] | 6.375% notes due 2037 (6.375% 2037 Notes) [Member]
Debt Instrument [Line Items]
Debt conversion, converted instrument, amount	348,000,000
Notes payable to banks [Member] | 6.90% notes due 2038 (6.90% 2038 Notes) [Member]
Debt Instrument [Line Items]
Debt conversion, converted instrument, amount	209,000,000
Notes payable to banks [Member] | 6.40% notes due 2039 (6.40% 2039 Notes) [Member]
Debt Instrument [Line Items]
Debt conversion, converted instrument, amount	534,000,000
Notes payable to banks [Member] | 5.75% notes due 2040 (5.75% 2040 Notes) [Member]
Debt Instrument [Line Items]
Debt conversion, converted instrument, amount	288,000,000
Notes payable to banks [Member] | 5.15% notes due 2041 (5.15% 2041 Notes) [Member]
Debt Instrument [Line Items]
Debt conversion, converted instrument, amount	1,276,000,000
Notes payable to banks [Member] | 5.65% notes due 2042 (5.65% 2042 Notes) [Member]
Debt Instrument [Line Items]
Debt conversion, converted instrument, amount	763,000,000
Notes payable to banks [Member] | 5.375% notes due 2043 (5.375% 2043 Notes) [Member]
Debt Instrument [Line Items]
Debt conversion, converted instrument, amount	739,000,000
Senior Notes [Member] | 4.563% notes due 2048 (4.563% 2048 Notes) [Member]
Debt Instrument [Line Items]
Aggregate principal amount of notes issued	$ 1,400,000,000
Stated contractual interest rate on note	4.563%
Senior Notes [Member] | 4.663% notes due 2051 (4.663% 2051 Notes) [Member]
Debt Instrument [Line Items]
Aggregate principal amount of notes issued	$ 3,500,000,000
Stated contractual interest rate on note	4.663%
Nonoperating Income (Expense) [Member] | Notes payable to banks [Member] | New senior notes [Member]
Debt Instrument [Line Items]
Debt related commitment fees and debt issuance costs	$ 24,000,000