Debt (Details) $ in Millions		8 Months Ended	12 Months Ended
	May 30, 2018 CAD ($)	Aug. 31, 2018 CAD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2016 CAD ($)	Jun. 14, 2018 CAD ($)
Line of Credit Facility [Line Items]
Write off of deferred financing costs			$ 60.5	$ 0.0	$ 0.0
Repayments of Debt			232.7	337.3	$ 0.0
Credit Facility, Stand by fees			1.1	0.7
2017 credit facility [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit				0.0
Write off of deferred financing costs	$ 60.5
Repayments of Debt	100.0
TMPL credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity						$ 1,000.0
2018 Credit facility [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit			0.0
Line of Credit Facility, Maximum Borrowing Capacity			$ 500.0
Debt Instrument, Term			4 years
Maximum ratio of consolidated total funded debt to consolidated earnings before interest income taxes, DDA and non-cash adjustments			5.00
Minimum ratio of consolidated total funded debt to consolidated earnings before interest income taxes DDA			1.00
Line of Credit Facility, Current Borrowing Capacity			$ 500.0
Letters of Credit Outstanding, Amount			11.0
Line of Credit Facility, Remaining Borrowing Capacity			$ 489.0
2018 Credit facility [Member] | Prime Rate
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.25%
2018 Temporary Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 500.0
Repayments of Debt			$ 132.6
Minimum [Member] | 2018 Credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage			0.20%
Minimum [Member] | 2018 Credit facility [Member] | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.00%
Maximum [Member] | 2018 Credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage			0.45%
Maximum [Member] | 2018 Credit facility [Member] | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate			2.25%
Trans Mountain Transaction [Member] | Letters of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity			$ 7.9
Trans Mountain Transaction [Member] | Discontinued Operations [Member]
Line of Credit Facility [Line Items]
Write off of deferred financing costs		$ 60.5
Credit Facility		$ 559.8		$ 0.0