Employee Benefit Plans (Reconciliation of Assets Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 15	$ 4
Purchases	33	24
Sales		(1)
Total gains (losses) and other, net		(21)
Ending balance	(161)	15
Master Trust [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	31	34
Sales	(2)	(2)
Total gains (losses) and other, net	0	(1)
Ending balance	38	$ 31
Piedmont Natural Gas [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchases	(187)
Piedmont Natural Gas [Member] | Master Trust [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Ending balance	$ (9)