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Commitments and Contingencies - Additional Information (Detail) $ in Thousands, CAD in Millions		3 Months Ended	6 Months Ended
	Jun. 16, 2017 CAD MW	Mar. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jul. 27, 2017 USD ($)	Apr. 21, 2017
Other Commitments [Line Items]
Other Commitment			$ 1,984
Change In Other Commitment		$ 102,200
Interest in Western Interconnect by third party [Member]
Other Commitments [Line Items]
Ownership Percentage						1.00%
Land [Member]
Other Commitments [Line Items]
Operating Lease, Expense			7,200	$ 6,700
Transmission Service Agreement [Member]
Other Commitments [Line Items]
Other Commitment			651,091
Operating Lease, Cost			$ 4,600
Transmission Service Agreement [Member] \| Minimum [Member]
Other Commitments [Line Items]
Lessee, Operating Lease, Term of Contract			25 years
Transmission Service Agreement [Member] \| Maximum [Member]
Other Commitments [Line Items]
Lessee, Operating Lease, Term of Contract			30 years
Meikle Wind Energy Limited Partnership [Member]
Other Commitments [Line Items]
Ownership Percentage	70.00%
Limited Partner Interest In Newly-formed Limited Partnership [Member]
Other Commitments [Line Items]
Ownership Percentage	50.99%
Pattern MSM GP Holdings, Inc. [Member]
Other Commitments [Line Items]
Ownership Percentage	70.00%
Pattern Development MSM Management ULC [Member]
Other Commitments [Line Items]
Ownership Percentage	70.00%
Power Sale Agreements [Member]
Other Commitments [Line Items]
Letters of credit issued			$ 153,100
Power Sale Agreements [Member] \| Minimum [Member]
Other Commitments [Line Items]
Termination Date			2019
Power Sale Agreements [Member] \| Maximum [Member]
Other Commitments [Line Items]
Termination Date			2039
Project Finance Agreements [Member]
Other Commitments [Line Items]
Letters of credit issued			$ 117,600
Turbine Availability Warranty and Service Guarantee [Member]
Other Commitments [Line Items]
Loss contingency accrual			2,400
Meikle Wind Energy Limited Partnership [Member]
Other Commitments [Line Items]
Ownership Percentage	50.99%
Payments to Acquire Businesses, Gross \| CAD	CAD 85.4
Generating Capacity \| MW	179
Meikle Wind Energy Limited Partnership [Member] \| Miekle Project Company [Member]
Other Commitments [Line Items]
Equity Method Investment, Ownership Percentage	0.02%
Mont Sainte-Marguerite Wind Farm LP [Member]
Other Commitments [Line Items]
Payments to Acquire Businesses, Gross \| CAD	CAD 53.0
Generating Capacity \| MW	143
Mont Sainte-Marguerite Wind Farm LP [Member] \| Limited Partner Interest In Newly-formed Limited Partnership [Member]
Other Commitments [Line Items]
Equity Method Investment, Ownership Percentage	100.00%
Pattern Development 2.0 [Member] \| Subsequent Event [Member]
Other Commitments [Line Items]
Equity Method Investment, Ownership Percentage					20.00%
Purchase Agreement, Maximum Potential Equity Ownership Percentage					29.00%
Pattern Development 2.0 [Member] \| Equity Method Investments [Member]
Other Commitments [Line Items]
Other Commitment			$ 240,000
Pattern Development 2.0 [Member] \| Equity Method Investments [Member] \| Subsequent Event [Member]
Other Commitments [Line Items]
Other Commitment					$ 60,000