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Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning $ 3,658 $ 4,055 $ 4,119
Other comprehensive income (loss), net of tax 51 (143) 196
Balance, ending 5,614 3,658 4,055
Pension Benefits [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Actuarial gains (losses) (29) (20)  
Defined Benefit Plans [Member] | Pension Benefits [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning (532) (412) (698)
Other comprehensive income (loss) before reclassifications (81) (140) 205
Amounts reclassified from accumulated other comprehensive income (loss) 29 20 37
Actuarial gains (losses)     44
Net current-period other comprehensive income (loss) (52) (120) 286
Balance, ending (584) (532) (412)
Defined Benefit Plans [Member] | Postretirement Costs [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning (119) (5) (74)
Other comprehensive income (loss) before reclassifications 136 (113) 67
Amounts reclassified from accumulated other comprehensive income (loss) 3 (1) 2
Net current-period other comprehensive income (loss) 139 (114) 69
Balance, ending 20 (119) (5)
Defined Benefit Plans [Member] | Deferred Taxes On Pension And OPEB Costs [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning 247 156 288
Other comprehensive income (loss) before reclassifications (24) 98 (100)
Amounts reclassified from accumulated other comprehensive income (loss) (12) (7) (15)
Actuarial gains (losses)     (17)
Net current-period other comprehensive income (loss) (36) 91 (132)
Balance, ending 211 247 156
Accumulated Other Comprehensive Loss [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning (404) (261) (484)
Other comprehensive income (loss) before reclassifications 31 (155) 172
Amounts reclassified from accumulated other comprehensive income (loss) 20 12 24
Actuarial gains (losses)     27
Net current-period other comprehensive income (loss) 51 (143) 223
Other comprehensive income (loss), net of tax 51 (143) 196
Balance, ending $ (353) $ (404) $ (261)