CONSOLIDATED STATEMENTS OF INCOME shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares		Dec. 31, 2016 USD ($) $ / shares shares		Dec. 31, 2015 USD ($) $ / shares shares
Transmission revenues	$ 422		$ 426	[1],[2]	$ 417	[1]
Equity earnings (Note 5)	124		97	[1]	97	[1]
Impairment of equity-method investment (Note 5)					(199)	[1]
Operation and maintenance expenses	(67)		(58)	[1],[2]	(61)	[1]
Property taxes	(28)		(27)	[1],[2]	(27)	[1]
General and administrative	(8)		(7)	[1],[2]	(9)	[1]
Depreciation	(97)		(96)	[1]	(95)	[1]
Financial charges and other (Note 12)	(82)		(71)	[1]	(63)	[1]
Net income before taxes	264		264	[1],[2]	60	[1]
Income taxes (Note 24)	(1)		(1)	[1],[2]	(2)	[1]
Net Income	263	[3]	263	[2]	58	[2]
Net income attributable to non-controlling interests	11		15	[1],[2]	21	[1]
Net income attributable to controlling interests	252		248	[1]	37	[1]
Net income attributable to controlling interest allocation (Note 13)
General Partner	16		11	[1],[2]	3	[1]
TransCanada and its subsidiaries	17		26	[1],[2]	36	[1]
Net income attributable to controlling interests	252		248	[1]	37	[1]
Common Units
Net income attributable to controlling interest allocation (Note 13)
Net income (loss) attributable to common units	$ 219		$ 211	[1]	$ (2)	[1]
Net income per common unit (Note 13) - basic (in dollars per unit) | $ / shares	$ 3.16	[4]	$ 3.21	[1],[4]	$ (0.03)	[1],[4]
Net income per common unit (Note 13) - diluted (in dollars per unit) | $ / shares	$ 3.16		$ 3.21		$ (0.03)
Weighted average common units outstanding - basic (in units) | shares	69.2		65.7	[1]	63.9	[1]
Weighted average common units outstanding - diluted (in units) | shares	69.2		65.7		63.9
Common units outstanding, end of year (in units) | shares	70.6		67.4	[1],[2]	64.3	[1]

[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).

[2] Recast to consolidate PNGTS (Refer to Notes 2 and 7).

[3] Recast to consolidate PNGTS (Refer to Notes 2 and 7).

[4] Net income per common unit prior to recast (Refer to Note 2).