EQUITY INVESTMENTS (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended			9 Months Ended
	Aug. 31, 2016	Apr. 30, 2006	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jan. 01, 2016	Dec. 31, 2015
EQUITY INVESTMENTS
Equity Earnings			$ 24		$ 17	$ 88	$ 63
Equity Investments			1,042			1,042			$ 965
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt			27			27			14
Amount borrowed						200	598
Northern Border
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt	$ 100								100
Northern Border | Revolving credit facility | Revolving Credit Agreement Expiring 2020
LIABILITIES AND PARTNERS' EQUITY
Amount borrowed	$ 200
Great Lakes
EQUITY INVESTMENTS
Total cash call issued to fund debt repayment				$ 9
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt			$ 19			$ 19			19
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)			50.00%			50.00%
Equity Earnings			$ 18		16	$ 52	50
Equity Investments			449			449			480
Amortization period of transaction fee		12 years
Transaction fee		$ 10
Additional ownership interest acquired (as a percent)		20.00%
Assets
Cash and cash equivalents			17			17			27
Other current assets			35			35			33
Plant, property and equipment, net			1,100			1,100			1,124
Other assets			15			15			16
Assets, total			1,167			1,167			1,200
LIABILITIES AND PARTNERS' EQUITY
Current liabilities			43			43			39
Deferred credits and other			28			28			26
Long-term debt, net			430			430			409
Partners' equity
Partners' equity			668			668			728
Accumulated other comprehensive loss			(2)			(2)			(2)
Liabilities and Partners' Equity, total			1,167			1,167			1,200
Revenues (expenses)
Transmission revenues			74		71	218	215
Operating expenses			(18)		(17)	(53)	(51)
Depreciation			(15)		(16)	(44)	(45)
Financial charges and other			(5)		(6)	(16)	(17)
Net income			$ 36		32	$ 105	102
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)			46.45%	46.45%		46.45%
Equity Earnings			$ 4		1	$ 23	13
Equity Investments			470			470			485
Equity contribution				$ 4		4	4
Assets
Current assets			48			48			86
Plant, property and equipment, net			718			718			727
Assets, total			766			766			813
LIABILITIES AND PARTNERS' EQUITY
Current liabilities			25			25			31
Long-term debt, net			288			288			297
Partners' equity
Partners' equity			453			453			485
Liabilities and Partners' Equity, total			766			766			813
Revenues (expenses)
Transmission revenues			36		29	133	105
Operating expenses			(15)		(15)	(45)	(40)
Depreciation			(7)		(7)	(21)	(21)
Financial charges and other			(6)		(5)	(17)	(17)
Net income			$ 8		$ 2	$ 50	$ 27
Portland Natural Gas Transmission System
EQUITY INVESTMENTS
Ownership interest (as a percent)			49.90%			49.90%		49.90%
Equity Earnings			$ 2			$ 13
Equity Investments			123			123
Undistributed earnings			$ 0			0
Equity contribution						$ 193
ASU 2015-03, Interest - Imputation of Interest | Adjustment | Northern Border
Assets
Other assets									(2)
LIABILITIES AND PARTNERS' EQUITY
Long-term debt, net									$ (2)