Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	$ 5,614	$ 3,658	$ 4,055
Other comprehensive income (loss) before reclassifications	(105)
Balance, ending	4,519	5,614	3,658
Defined Benefit Plans [Member] | Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	(584)	(532)	(412)
Other comprehensive income (loss) before reclassifications	(103)	(81)	(140)
Amounts reclassified from accumulated other comprehensive income (loss)	40	29	20
Net current-period other comprehensive income (loss)	(63)	(52)	(120)
Balance, ending	(647)	(584)	(532)
Defined Benefit Plans [Member] | Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	20	(119)	(5)
Other comprehensive income (loss) before reclassifications	17	136	(113)
Amounts reclassified from accumulated other comprehensive income (loss)	(8)	3	(1)
Net current-period other comprehensive income (loss)	9	139	(114)
Balance, ending	29	20	(119)
Defined Benefit Plans [Member] | Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	211	247	156
Other comprehensive income (loss) before reclassifications	32	(24)	98
Amounts reclassified from accumulated other comprehensive income (loss)	(12)	(12)	(7)
Net current-period other comprehensive income (loss)	20	(36)	91
Balance, ending	231	211	247
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	(353)	(404)	(261)
Other comprehensive income (loss) before reclassifications	(54)	31	(155)
Amounts reclassified from accumulated other comprehensive income (loss)	20	20	12
Net current-period other comprehensive income (loss)	(34)	51	(143)
Balance, ending	$ (387)	$ (353)	$ (404)