EQUITY INVESTMENTS (Details) - USD ($) $ in Millions						1 Months Ended	3 Months Ended				9 Months Ended
		Sep. 01, 2017	Aug. 01, 2017	Jul. 27, 2017	Jun. 01, 2017	Apr. 30, 2006	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016		Nov. 01, 2017	Oct. 31, 2017	Dec. 31, 2016
EQUITY INVESTMENTS
Equity Earnings							$ 27.0		$ 22.0	[1]	$ 87.0	$ 75.0	[1]
Equity Investments							$ 1,207.0				1,207.0					$ 918.0	[1]
Partnership distribution	[2]										235.0
Partnership's share of distributions											106.0	125.0	[1]
Return on investment distribution classified as investing activities											3.0
Great Lakes
EQUITY INVESTMENTS
Total cash call issued to fund debt repayment											$ 9.0
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)		50.00%					50.00%				50.00%				50.00%
Equity Earnings							$ 16.0		18.0		$ 50.0	52.0
Equity Investments							516.0				516.0					444.0
Amortization period of transaction fee						12 years
Transaction fee						$ 10.0
Additional ownership interest acquired (as a percent)						20.00%
Equity contribution		$ 83.0									83.0
Capital contribution to reduce the outstanding balance of revolver debt		$ 166.0
Assets
Cash and cash equivalents							24.0				24.0					14.0
Other current assets							36.0				36.0					36.0
Plant, property and equipment, net							1,069.0				1,069.0					1,089.0
Other assets							14.0				14.0					14.0
Assets, total							1,143.0				1,143.0					1,153.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities							48.0				48.0					38.0
Deferred credits and other							30.0				30.0					28.0
Long-term debt, including current maturities, net							264.0				264.0					430.0
Partners' equity
Partners' equity							802.0				802.0					659.0
Accumulated other comprehensive loss							(1.0)				(1.0)					(2.0)
Liabilities and Partners' Equity, total							1,143.0				1,143.0					1,153.0
Revenues (expenses)
Transmission revenues							73.0		74.0		217.0	218.0
Operating expenses							(20.0)		(18.0)		(56.0)	(53.0)
Depreciation							(15.0)		(15.0)		(45.0)	(44.0)
Financial charges and other							(5.0)		(5.0)		(14.0)	(16.0)
Net income							$ 33.0		36.0		$ 102.0	105.0
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)							46.45%				46.45%			46.45%
Equity Earnings							$ 2.0		4.0		$ 24.0	23.0
Equity Investments							469.0				469.0					474.0
Equity contribution								$ 4.0			4.0	4.0	[1]
Assets
Current assets							56.0				56.0					66.0
Plant, property and equipment, net							705.0				705.0					714.0
Assets, total							761.0				761.0					780.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities							41.0				41.0					40.0
Long-term debt, including current maturities, net							269.0				269.0					278.0
Partners' equity
Partners' equity							451.0				451.0					462.0
Liabilities and Partners' Equity, total							761.0				761.0					780.0
Revenues (expenses)
Transmission revenues							34.0		36.0		138.0	133.0
Operating expenses							(19.0)		(15.0)		(49.0)	(45.0)
Depreciation							(7.0)		(7.0)		(21.0)	(21.0)
Financial charges and other							(5.0)		(6.0)		(16.0)	(17.0)
Net income							$ 3.0		8.0		$ 52.0	50.0
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)			49.34%		49.34%		49.34%				49.34%
Equity Earnings							$ 9.0				$ 13.0
Equity Investments							222.0				222.0
Equity contribution					$ 710.0
Undistributed earnings							0.0				0.0
Assets
Cash and cash equivalents							93.0				93.0					86.0
Other current assets							33.0				33.0					34.0
Plant, property and equipment, net							592.0				592.0					604.0
Other assets							8.0				8.0					7.0
Assets, total							726.0				726.0					731.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities							20.0				20.0					18.0
Long-term debt, including current maturities, net							332.0				332.0					335.0
Other non-current liabilities							9.0				9.0					6.0
Partners' equity
Partners' equity							365.0				365.0					372.0
Liabilities and Partners' Equity, total							726.0				726.0					$ 731.0
Revenues (expenses)
Transmission revenues							43.0		45.0		142.0	145.0
Operating expenses							(13.0)		(13.0)		(41.0)	(43.0)
Depreciation							(7.0)		(9.0)		(22.0)	(28.0)
Financial charges and other							(4.0)		(4.0)		(13.0)	(12.0)
Net income							$ 19.0		$ 19.0		$ 66.0	$ 62.0
Iroquois | Distribution declared
EQUITY INVESTMENTS
Partnership distribution				$ 28.0
Iroquois | Cash Distribution Paid
EQUITY INVESTMENTS
Partnership's share of distributions			$ 14.0
Return on investment distribution classified as investing activities			$ 2.6

[1] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).

[2] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).