Schedule I - Condensed Financial Information - Condensed Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current Assets
Cash and temporary cash investments	$ 797	$ 776	$ 944	$ 699
Taxes receivable	64	79
Prepayments	178	159
Other current assets	189	151
TOTAL CURRENT ASSETS	3,537	3,406
Investments in subsidiaries	2,001	1,921
Goodwill	428	428
Other noncurrent assets	4,973	7,712
TOTAL ASSETS	48,111	48,255	45,642
Current Liabilities
Long-term debt due within one year	1,298	39
Notes payable	577	1,054
Other current liabilities	386	297
TOTAL CURRENT LIABILITIES	4,902	3,843
Long-term debt	14,731	14,735
Shareholders’ Equity
Equity	15,418	14,298
TOTAL LIABILITIES AND EQUITY	48,111	48,255
Parent Company
Current Assets
Cash and temporary cash investments	9	13	$ 51	$ 3
Taxes receivable	45	79
Accounts receivable from affiliated companies	687	702
Prepayments	36	24
Other current assets	18	19
TOTAL CURRENT ASSETS	795	837
Investments in subsidiaries	15,110	13,991
Goodwill	406	406
Deferred income tax	18	42
Long term debt receivable from affiliated companies	900	900
Other noncurrent assets	2	16
TOTAL ASSETS	17,231	16,192
Current Liabilities
Long-term debt due within one year	2	2
Notes payable	331	384
Accounts payable to affiliated companies	274	288
Other current liabilities	10	22
TOTAL CURRENT LIABILITIES	617	696
Total Liabilities	617	696
Long-term debt	1,195	1,198
Shareholders’ Equity
Common stock, including additional paid-in capital	6,331	5,887
Retained earnings	9,088	8,411
Equity	15,419	14,298
TOTAL LIABILITIES AND EQUITY	$ 17,231	$ 16,192