CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) shares in Thousands, $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Accounts receivable, allowance	$ 168	$ 164
PP&E, accumulated depreciation	$ 25,116	$ 24,352
Preferred stock-non-redeemable, par value (in dollars per share)	$ 25.00	$ 25.00
Preferred stock-non-redeemable, shares authorized (in shares)	2,000	2,000
Preferred stock-non-redeemable, shares issued (in shares)	7	7
Preferred stock-non-redeemable, shares outstanding (in shares)	7	7
Common stock, par value (in dollars per share)	$ 1.00	$ 1.00
Common stock, shares authorized (in shares)	1,600,000	1,600,000
Common stock, shares issued (in shares)	1,078,632	1,069,169
Common stock, shares outstanding (in shares)	1,078,632	1,069,169
Customer relationships
Finite-lived intangible assets, accumulated amortization	$ 7,450	$ 7,096
Other intangible assets
Finite-lived intangible assets, accumulated amortization	$ 2,412	$ 2,325