Hydro One 40-F 27 Mar 17 2016 FY Annual report (Canada) Financial data

Document and Entity Information

Document and Entity Information	12 Months Ended
	Dec. 31, 2016shares
Document And Entity Information [Abstract]
Document Type	40-F
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
Entity Registrant Name	HYDRO ONE INC
Entity Central Index Key	1,114,445
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Common Stock, Shares Outstanding	142,239

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Income - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Revenues
Distribution (includes $160 related party revenues; 2015 - $159) (Note 26)	CAD 4,915	CAD 4,949
Transmission (includes $1,556 related party revenues; 2015 - $1,554) (Note 26)	1,587	1,536
Other		44
Total revenues	6,502	6,529
Costs
Purchased power (includes $2,103 related party costs; 2015 - $2,335) (Note 26)	3,427	3,450
Operation, maintenance and administration (Note 26)	1,043	1,130
Depreciation and amortization (Note 5)	769	757
Total costs	5,239	5,337
Income (loss) before financing charges and income taxes	1,263	1,192
Financing charges (Note 6)	392	376
Income before income taxes	871	816
Income taxes (Notes 7, 26)	135	114
Net income	736	702
Other comprehensive income	0	0
Comprehensive income	736	702
Net income and comprehensive income attributable to:
Noncontrolling interest (Note 25)	6	10
Preferred shareholder		13
Common shareholder	730	679
Net income	CAD 736	CAD 702
Earnings per common share (Note 23)
Basic	CAD 5,132	CAD 6,340
Diluted	5,132	6,340
Dividends per common share declared (Note 22)	CAD 14	CAD 8,750

Consolidated Statements of Ope3

Consolidated Statements of Operations and Comprehensive Income (Parenthetical) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Distribution revenues	CAD 4,915	CAD 4,949
Transmission revenues	1,587	1,536
Purchased power costs	3,427	3,450
Related Party [Member]
Distribution revenues	160	159
Transmission revenues	1,556	1,554
Purchased power costs	CAD 2,103	CAD 2,335

Consolidated Balance Sheets

Consolidated Balance Sheets - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	CAD 48	CAD 89
Accounts receivable (Note 8)	833	772
Due from related parties (Note 26)	224	184
Other current assets (Note 9)	97	100
Total current assets	1,202	1,145
Property, plant and equipment (Note 10)	19,068	17,893
Other long-term assets:
Regulatory assets (Note 12)	3,145	3,015
Deferred income tax assets (Note 7)	1,213	1,610
Intangible assets (Note 11)	349	336
Goodwill (Note 4)	327	163
Other assets	6	7
Total other long-term assets	5,040	5,131
Total assets	25,310	24,169
Current liabilities:
Short-term notes payable (Note 15)	469	1,491
Long-term debt payable within one year (Note 15)	602	500
Accounts payable and other current liabilities (Note 13)	933	858
Due to related parties (Note 26)	253	132
Total current liabilities	2,257	2,981
Long-term liabilities:
Long-term debt (includes $548 measured at fair value; 2015 - $51) (Notes 15, 16)	10,078	8,207
Regulatory liabilities (Note 12)	209	236
Deferred income tax liabilities (Note 7)	60	206
Other long-term liabilities (Note 14)	2,765	2,714
Total Long-term liabilities	13,112	11,363
Total liabilities	15,369	14,344
Contingencies and Commitments (Notes 28, 29)
Subsequent Events (Note 31)	0	0
Noncontrolling interest subject to redemption (Note 25)	22	23
Equity
Common shares (Notes 21, 22)	5,391	6,000
Retained earnings	4,487	3,759
Accumulated other comprehensive loss	(9)	(9)
Hydro One shareholder's equity	9,869	9,750
Noncontrolling interest (Note 25)	50	52
Total equity	9,919	9,802
Total liabilities, preferred shares, noncontrolling interest subject to redemption and equity	CAD 25,310	CAD 24,169

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Long-term debt measured at fair value	CAD 548	CAD 51

Consolidated Statements of Chan

Consolidated Statements of Changes in Equity - CAD CAD in Millions	Total	Common Shares [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Hydro One [Member]	Non-controlling Interest (Note 25) [Member]
Beginning balance at Dec. 31, 2014	CAD 7,603	CAD 3,314	CAD 4,249	CAD (9)	CAD 7,554	CAD 49
Net income	699		692		692	7
Other comprehensive income	0	0	0	0	0	0
Distributions to noncontrolling interest	(4)					(4)
Dividends on preferred shares	(13)		(13)		(13)
Dividends on common shares	(875)		(875)		(875)
Common shares issued (Note 21)	2,923	2,923			2,923
Return of stated capital (Note 21)	0
Hydro One Brampton spin-off (Note 4)	(454)	(196)	(258)		(454)
Hydro One Telecom and MBSI spin-offs (Note 4)	(77)	(41)	(36)		(77)
Ending balance at Dec. 31, 2015	9,802	6,000	3,759	(9)	9,750	52
Net income	734		730		730	4
Other comprehensive income	0	0	0	0	0	0
Distributions to noncontrolling interest	(6)					(6)
Dividends on common shares	(2)		(2)		(2)
Return of stated capital (Note 21)	(609)	(609)			(609)
Ending balance at Dec. 31, 2016	CAD 9,919	CAD 5,391	CAD 4,487	CAD (9)	CAD 9,869	CAD 50

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net income	CAD 736	CAD 702
Environmental expenditures	(20)	(19)
Adjustments for non-cash items:
Depreciation and amortization (excluding removal costs)	679	667
Regulatory assets and liabilities	(16)	(3)
Deferred income taxes (Note 7)	111	(2,817)
Other	10	24
Changes in non-cash balances related to operations (Note 27)	168	192
Net cash from (used in) operating activities	1,668	(1,254)
Financing activities
Long-term debt issued	2,300	350
Long-term debt repaid	(502)	(585)
Short-term notes issued	3,031	2,891
Short-term notes repaid	(4,053)	(1,400)
Common shares issued		2,600
Return of stated capital	(609)
Dividends paid	(2)	(888)
Distributions paid to noncontrolling interest	(9)	(5)
Change in bank indebtedness		(2)
Other	(10)	(7)
Net cash from financing activities	146	2,954
Investing activities
Property, plant and equipment	(1,594)	(1,594)
Intangible assets	(61)	(37)
Capital contributions received (Note 27)	21	57
Acquisitions (Note 4)	(224)	(90)
Other	3	6
Net cash used in investing activities	(1,855)	(1,711)
Net change in cash and cash equivalents	(41)	(11)
Cash and cash equivalents, beginning of year	89	100
Cash and cash equivalents, end of year	CAD 48	89
Hydro One Brampton [Member]
Investing activities
Investment in Hydro One Brampton (Note 4)		CAD (53)

Description of the Business

Description of the Business	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Description of the Business	1. DESCRIPTION OF THE BUSINESS Hydro One Inc. (Hydro One or the Company) was incorporated on December 1, 1998, under the Business Corporations Act On October 31, 2015, Hydro One Limited, a subsidiary of the Province of Ontario (Province), acquired Hydro One. The principal businesses of Hydro One are the transmission and distribution of electricity to customers within Ontario. In November 2015, Hydro One Limited and the Province completed an initial public offering (IPO) on the Toronto Stock Exchange. See note 21 for further details regarding the reorganization of Hydro One.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Significant Accounting Policies	2. SIGNIFICANT ACCOUNTING POLICIES Basis of Consolidation These Consolidated Financial Statements include the accounts of the Company and its subsidiaries. Intercompany transactions and balances have been eliminated. Basis of Accounting These Consolidated Financial Statements are prepared and presented in accordance with United States (US) Generally Accepted Accounting Principles (GAAP) and in Canadian dollars. Use of Management Estimates The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities at the date of the financial statements and the reported amounts of revenues, expenses, gains and losses during the reporting periods. Management evaluates these estimates on an ongoing basis based upon historical experience, current conditions, and assumptions believed to be reasonable at the time the assumptions are made, with any adjustments being recognized in results of operations in the period they arise. Significant estimates relate to regulatory assets and regulatory liabilities, environmental liabilities, pension benefits, post-retirement and post-employment benefits, asset retirement obligations, goodwill and asset impairments, contingencies, unbilled revenues, allowance for doubtful accounts, derivative instruments, and deferred income tax assets and liabilities. Actual results may differ significantly from these estimates. Rate Setting The Company’s Transmission Business includes the transmission business of Hydro One Networks Inc. (Hydro One Networks), Hydro One Sault Ste. Marie LP (previously Great Lakes Power Transmission LP (Great Lakes Power)), and its 66% interest in B2M Limited Partnership (B2M LP). The Company’s Distribution Business includes the distribution businesses of Hydro One Networks, as well as Hydro One Remote Communities Inc. (Hydro One Remote Communities). Transmission In November 2015, the OEB approved Hydro One Networks’ 2016 transmission rates revenue requirement of $1,480 million. In December 2015, the OEB approved B2M LP’s 2015-2019 rates revenue requirements of $39 million, $36 million, $37 million, $38 million and $37 million for the respective years. On January 14, 2016, the OEB approved the B2M LP revenue requirement recovery through the 2016 Uniform Transmission Rates, and the establishment of a deferral account to capture costs of Tax Rate and Rule changes. Distribution In March 2015, the OEB approved Hydro One Networks’ distribution revenue requirements of $1,326 million for 2015, $1,430 million for 2016 and $1,486 million for 2017. The OEB has subsequently approved updated revenue requirements of $1,410 million for 2016 and $1,415 million for 2017. On March 17, 2016, the OEB approved an increase of 2.10% to Hydro One Remote Communities’ basic rates for the distribution and generation of electricity, with an effective date of May 1, 2016. Regulatory Accounting The OEB has the general power to include or exclude revenues, costs, gains or losses in the rates of a specific period, resulting in a change in the timing of accounting recognition from that which would have been applied in an unregulated company. Such change in timing involves the application of rate-regulated accounting, giving rise to the recognition of regulatory assets and liabilities. The Company’s regulatory assets represent certain amounts receivable from future customers and costs that have been deferred for accounting purposes because it is probable that they will be recovered in future rates. In addition, the Company has recorded regulatory liabilities that generally represent amounts that are refundable to future customers. The Company continually assesses the likelihood of recovery of each of its regulatory assets and continues to believe that it is probable that the OEB will include its regulatory assets and liabilities in setting of future rates. If, at some future date, the Company judges that it is no longer probable that the OEB will include a regulatory asset or liability in setting future rates, the appropriate carrying amount would be reflected in results of operations in the period that the assessment is made. Cash and Cash Equivalents Cash and cash equivalents include cash and short-term investments with an original maturity of three months or less. Revenue Recognition Transmission revenues are collected through OEB-approved rates, which are based on an approved revenue requirement that includes a rate of return. Such revenue is recognized as electricity is transmitted and delivered to customers. Distribution revenues attributable to the delivery of electricity are based on OEB-approved distribution rates and are recognized on an accrual basis and include billed and unbilled revenues. Billed revenues are based on electricity delivered as measured from customer meters. At the end of each month, electricity delivered to customers since the date of the last billed meter reading is estimated, and the corresponding unbilled revenue is recorded. The unbilled revenue estimate is affected by energy consumption, weather, and changes in the composition of customer classes. Distribution revenue also includes an amount relating to rate protection for rural, residential, and remote customers, which is received from the Independent Electricity System Operator (IESO) based on a standardized customer rate that is approved by the OEB. Revenues also include amounts related to sales of other services and equipment. Such revenue is recognized as services are rendered or as equipment is delivered. Revenues are recorded net of indirect taxes. Accounts Receivable and Allowance for Doubtful Accounts Billed accounts receivable are recorded at the invoiced amount, net of allowance for doubtful accounts. Unbilled accounts receivable are recorded at their estimated value. Overdue amounts related to regulated billings bear interest at OEB-approved rates. The allowance for doubtful accounts reflects the Company’s best estimate of losses on billed accounts receivable balances. The Company estimates the allowance for doubtful accounts on billed accounts receivable by applying internally developed loss rates to the outstanding receivable balances by aging category. Loss rates applied to the billed accounts receivable balances are based on historical overdue balances, customer payments and write-offs. Accounts receivable are written-off against the allowance when they are deemed uncollectible. The allowance for doubtful accounts is affected by changes in volume, prices and economic conditions. Noncontrolling interest Noncontrolling interest represents the portion of equity ownership in subsidiaries that is not attributable to shareholders of Hydro One. Noncontrolling interest is initially recorded at fair value and subsequently the amount is adjusted for the proportionate share of net income and other comprehensive income attributable to the noncontrolling interest and any dividends or distributions paid to the noncontrolling interest. If a transaction results in the acquisition of all, or part, of a noncontrolling interest in a subsidiary, the acquisition of the noncontrolling interest is accounted for as an equity transaction. No gain or loss is recognized in consolidated net income or comprehensive income as a result of changes in the noncontrolling interest, unless a change results in the loss of control by the Company. Income Taxes Prior to the IPO of Hydro One Limited, Hydro One was exempt from tax under the Income Tax Act Taxation Act, 2007 Electricity Act Current and deferred income taxes are computed based on the tax rates and tax laws enacted as at the balance sheet date. Tax benefits associated with income tax positions taken, or expected to be taken, in a tax return are recorded only when the “more-likely-than-not” recognition threshold is satisfied and are measured at the largest amount of benefit that has a greater than 50% likelihood of being realized upon settlement. Management evaluates each position based solely on the technical merits and facts and circumstances of the position, assuming the position will be examined by a taxing authority having full knowledge of all relevant information. Significant management judgment is required to determine recognition thresholds and the related amount of tax benefits to be recognized in the Consolidated Financial Statements. Management re-evaluates tax positions each period using new information about recognition or measurement as it becomes available. Deferred Income Taxes Deferred income taxes are provided for using the liability method. Deferred income taxes are recognized based on the estimated future tax consequences attributable to temporary differences between the carrying amount of assets and liabilities in the Consolidated Financial Statements and their corresponding tax bases. Deferred income tax liabilities are recognized on all taxable temporary differences. Deferred tax assets are recognized to the extent that it is more-likely-than-not that these assets will be realized from taxable income available against which deductible temporary differences can be utilized. Deferred income taxes are calculated at the tax rates that are expected to apply in the period when the liability is settled or the asset is realized, based on the tax rates and tax laws that have been enacted as at the balance sheet date. Deferred income taxes that are not included in the rate-setting process are charged or credited to the Consolidated Statements of Operations and Comprehensive Income. If management determines that it is more-likely-than-not that some or all of a deferred income tax asset will not be realized, a valuation allowance is recorded against the deferred income tax asset to report the net balance at the amount expected to be realized. Previously unrecognized deferred income tax assets are reassessed at each balance sheet date and are recognized to the extent that it has become more-likely-than-not that the tax benefit will be realized. The Company records regulatory assets and liabilities associated with deferred income taxes that will be included in the rate-setting process. The Company uses the flow-through method to account for investment tax credits (ITCs) earned on eligible scientific research and experimental development expenditures, and apprenticeship job creation. Under this method, only non-refundable ITCs are recognized as a reduction to income tax expense. Materials and Supplies Materials and supplies represent consumables, small spare parts and construction materials held for internal construction and maintenance of property, plant and equipment. These assets are carried at average cost less any impairments recorded. Property, Plant and Equipment Property, plant and equipment is recorded at original cost, net of customer contributions, and any accumulated impairment losses. The cost of additions, including betterments and replacement asset components, is included on the Consolidated Balance Sheets as property, plant and equipment. The original cost of property, plant and equipment includes direct materials, direct labour (including employee benefits), contracted services, attributable capitalized financing costs, asset retirement costs, and direct and indirect overheads that are related to the capital project or program. Indirect overheads include a portion of corporate costs such as finance, treasury, human resources, information technology and executive costs. Overhead costs, including corporate functions and field services costs, are capitalized on a fully allocated basis, consistent with an OEB-approved methodology. Property, plant and equipment in service consists of transmission, distribution, communication, administration and service assets and land easements. Property, plant and equipment also includes future use assets, such as land, major components and spare parts, and capitalized project development costs associated with deferred capital projects. Transmission Transmission assets include assets used for the transmission of high-voltage electricity, such as transmission lines, support structures, foundations, insulators, connecting hardware and grounding systems, and assets used to step up the voltage of electricity from generating stations for transmission and to step down voltages for distribution, including transformers, circuit breakers and switches. Distribution Distribution assets include assets related to the distribution of low-voltage electricity, including lines, poles, switches, transformers, protective devices and metering systems. Communication Communication assets include fibre optic and microwave radio systems, optical ground wire, towers, telephone equipment and associated buildings. Administration and Service Administration and service assets include administrative buildings, personal computers, transport and work equipment, tools and other minor assets. Easements Easements include statutory rights of use for transmission corridors and abutting lands granted under the Reliable Energy and Consumer Protection Act, 2002 Intangible Assets Intangible assets separately acquired or internally developed are measured on initial recognition at cost, which comprises purchased software, direct labour (including employee benefits), consulting, engineering, overheads and attributable capitalized financing charges. Following initial recognition, intangible assets are carried at cost, net of any accumulated amortization and accumulated impairment losses. The Company’s intangible assets primarily represent major computer applications. Capitalized Financing Costs Capitalized financing costs represent interest costs attributable to the construction of property, plant and equipment or development of intangible assets. The financing cost of attributable borrowed funds is capitalized as part of the acquisition cost of such assets. The capitalized financing costs are a reduction of financing charges recognized in the Consolidated Statements of Operations and Comprehensive Income. Capitalized financing costs are calculated using the Company’s weighted average effective cost of debt. Construction and Development in Progress Construction and development in progress consists of the capitalized cost of constructed assets that are not yet complete and which have not yet been placed in service. Depreciation and Amortization The cost of property, plant and equipment and intangible assets is depreciated or amortized on a straight-line basis based on the estimated remaining service life of each asset category, except for transport and work equipment, which is depreciated on a declining balance basis. The Company periodically initiates an external independent review of its property, plant and equipment and intangible asset depreciation and amortization rates, as required by the OEB. Any changes arising from OEB approval of such a review are implemented on a remaining service life basis, consistent with their inclusion in electricity rates. The last review resulted in changes to rates effective January 1, 2015. A summary of average service lives and depreciation and amortization rates for the various classes of assets is included below: Average Rate Service Life Range Average Property, plant and equipment: Transmission 56 years 1% – 3 % 2 % Distribution 46 years 1% – 7 % 2 % Communication 16 years 1% – 15 % 6 % Administration and service 18 years 1% – 20 % 7 % Intangible assets 10 years 10 % 10 % In accordance with group depreciation practices, the original cost of property, plant and equipment, or major components thereof, and intangible assets that are normally retired, is charged to accumulated depreciation, with no gain or loss being reflected in results of operations. Where a disposition of property, plant and equipment occurs through sale, a gain or loss is calculated based on proceeds and such gain or loss is included in depreciation expense. Acquisitions and Goodwill The Company accounts for business acquisitions using the acquisition method of accounting and, accordingly, the assets and liabilities of the acquired entities are primarily measured at their estimated fair value at the date of acquisition. Goodwill represents the cost of acquired companies that is in excess of the fair value of the net identifiable assets acquired at the acquisition date. Goodwill is not included in rate base. Goodwill is evaluated for impairment on an annual basis, or more frequently if circumstances require. The Company performs a qualitative assessment to determine whether it is more-likely-than-not that the fair value of the applicable reporting unit is less than its carrying amount. If the Company determines, as a result of its qualitative assessment, that it is not more-likely-than-not that the fair value of the applicable reporting unit is less than its carrying amount, no further testing is required. If the Company determines, as a result of its qualitative assessment, that it is more-likely-than-not that the fair value of the applicable reporting unit is less than its carrying amount, a goodwill impairment assessment is performed using a two-step, fair value-based test. The first step compares the fair value of the applicable reporting unit to its carrying amount, including goodwill. If the carrying amount of the applicable reporting unit exceeds its fair value, a second step is performed. The second step requires an allocation of fair value to the individual assets and liabilities using purchase price allocation in order to determine the implied fair value of goodwill. If the implied fair value of goodwill is less than the carrying amount, an impairment loss is recorded as a reduction to goodwill and as a charge to results of operations. For the year ended December 31, 2016, based on the qualitative assessment performed as at September 30, 2016, the Company has determined that it is not more-likely-than-not that the fair value of each applicable reporting unit assessed is less than its carrying amount. As a result, no further testing was performed, and the Company has concluded that goodwill was not impaired at December 31, 2016. Long-Lived Asset Impairment When circumstances indicate the carrying value of long-lived assets may not be recoverable, the Company evaluates whether the carrying value of such assets, excluding goodwill, has been impaired. For such long-lived assets, the Company evaluates whether impairment may exist by estimating future estimated undiscounted cash flows expected to result from the use and eventual disposition of the asset. When alternative courses of action to recover the carrying amount of a long-lived asset are under consideration, a probability-weighted approach is used to develop estimates of future undiscounted cash flows. If the carrying value of the long-lived asset is not recoverable based on the estimated future undiscounted cash flows, an impairment loss is recorded, measured as the excess of the carrying value of the asset over its fair value. As a result, the asset’s carrying value is adjusted to its estimated fair value. Within its regulated business, the carrying costs of most of Hydro One’s long-lived assets are included in rate base where they earn an OEB-approved rate of return. Asset carrying values and the related return are recovered through approved rates. As a result, such assets are only tested for impairment in the event that the OEB disallows recovery, in whole or in part, or if such a disallowance is judged to be probable. As at December 31, 2016 and 2015, no asset impairment had been recorded. Costs of Arranging Debt Financing For financial liabilities classified as other than held-for-trading, the Company defers the external transaction costs related to obtaining debt financing and presents such amounts net of related debt on the Consolidated Balance Sheets. Deferred debt issuance costs are amortized over the contractual life of the related debt on an effective-interest basis and the amortization is included within financing charges in the Consolidated Statements of Operations and Comprehensive Income. Transaction costs for items classified as held-for-trading are expensed immediately. Comprehensive Income Comprehensive income is comprised of net income and other comprehensive income (OCI). Hydro One presents net income and OCI in a single continuous Consolidated Statement of Operations and Comprehensive Income. Financial Assets and Liabilities All financial assets and liabilities are classified into one of the following five categories: held-to-maturity; loans and receivables; held-for-trading; other liabilities; or available-for-sale. Financial assets and liabilities classified as held-for-trading are measured at fair value. All other financial assets and liabilities are measured at amortized cost, except accounts receivable and amounts due from related parties, which are measured at the lower of cost or fair value. Accounts receivable and amounts due from related parties are classified as loans and receivables. The Company considers the carrying amounts of accounts receivable and amounts due from related parties to be reasonable estimates of fair value because of the short time to maturity of these instruments. Provisions for impaired accounts receivable are recognized as adjustments to the allowance for doubtful accounts and are recognized when there is objective evidence that the Company will not be able to collect amounts according to the original terms. All financial instrument transactions are recorded at trade date. Derivative instruments are measured at fair value. Gains and losses from fair valuation are included within financing charges in the period in which they arise. The Company determines the classification of its financial assets and liabilities at the date of initial recognition. The Company designates certain of its financial assets and liabilities to be held at fair value, when it is consistent with the Company’s risk management policy disclosed in Note 16 – Fair Value of Financial Instruments and Risk Management. Derivative Instruments and Hedge Accounting The Company closely monitors the risks associated with changes in interest rates on its operations and, where appropriate, uses various instruments to hedge these risks. Certain of these derivative instruments qualify for hedge accounting and are designated as accounting hedges, while others either do not qualify as hedges or have not been designated as hedges (hereinafter referred to as undesignated contracts) as they are part of economic hedging relationships. The accounting guidance for derivative instruments requires the recognition of all derivative instruments not identified as meeting the normal purchase and sale exemption as either assets or liabilities recorded at fair value on the Consolidated Balance Sheets. For derivative instruments that qualify for hedge accounting, the Company may elect to designate such derivative instruments as either cash flow hedges or fair value hedges. The Company offsets fair value amounts recognized on its Consolidated Balance Sheets related to derivative instruments executed with the same counterparty under the same master netting agreement. For derivative instruments that qualify for hedge accounting and which are designated as cash flow hedges, the effective portion of any gain or loss, net of tax, is reported as a component of accumulated OCI (AOCI) and is reclassified to results of operations in the same period or periods during which the hedged transaction affects results of operations. Any gains or losses on the derivative instrument that represent either hedge ineffectiveness or hedge components excluded from the assessment of effectiveness are recognized in results of operations. For fair value hedges, changes in fair value of both the derivative instrument and the underlying hedged exposure are recognized in the Consolidated Statements of Operations and Comprehensive Income in the current period. The gain or loss on the derivative instrument is included in the same line item as the offsetting gain or loss on the hedged item in the Consolidated Statements of Operations and Comprehensive Income. The changes in fair value of the undesignated derivative instruments are reflected in results of operations. Embedded derivative instruments are separated from their host contracts and are carried at fair value on the Consolidated Balance Sheets when: (a) the economic characteristics and risks of the embedded derivative are not clearly and closely related to the economic characteristics and risks of the host contract; (b) the hybrid instrument is not measured at fair value, with changes in fair value recognized in results of operations each period; and (c) the embedded derivative itself meets the definition of a derivative. The Company does not engage in derivative trading or speculative activities and had no embedded derivatives at December 31, 2016 or 2015. Hydro One periodically develops hedging strategies taking into account risk management objectives. At the inception of a hedging relationship where the Company has elected to apply hedge accounting, Hydro One formally documents the relationship between the hedged item and the hedging instrument, the related risk management objective, the nature of the specific risk exposure being hedged, and the method for assessing the effectiveness of the hedging relationship. The Company also assesses, both at the inception of the hedge and on a quarterly basis, whether the hedging instruments are effective in offsetting changes in fair values or cash flows of the hedged items. Employee Future Benefits Employee future benefits provided by Hydro One include pension, post-retirement and post-employment benefits. The costs of the Company’s pension, post-retirement and post-employment benefit plans are recorded over the periods during which employees render service. The Company recognizes the funded status of its defined benefit pension, post-retirement and post-employment plans on its Consolidated Balance Sheets and subsequently recognizes the changes in funded status at the end of each reporting year. Defined benefit pension, post-retirement and post-employment plans are considered to be underfunded when the projected benefit obligation exceeds the fair value of the plan assets. Liabilities are recognized on the Consolidated Balance Sheets for any net underfunded projected benefit obligation. The net underfunded projected benefit obligation may be disclosed as a current liability, long-term liability, or both. The current portion is the amount by which the actuarial present value of benefits included in the benefit obligation payable in the next 12 months exceeds the fair value of plan assets. If the fair value of plan assets exceeds the projected benefit obligation of the plan, an asset is recognized equal to the net overfunded projected benefit obligation. The post-retirement and post-employment benefit plans are unfunded because there are no related plan assets. Hydro One recognizes its contributions to the defined contribution pension plan as pension expense, with a portion being capitalized as part of labour costs included in capital expenditures. The expensed amount is included in operation, maintenance and administration costs in the Consolidated Statements of Operations and Comprehensive Income. Defined Benefit Pension Defined benefit pension costs are recorded on an accrual basis for financial reporting purposes. Pension costs are actuarially determined using the projected benefit method prorated on service and are based on assumptions that reflect management’s best estimate of the effect of future events, including future compensation increases. Past service costs from plan amendments and all actuarial gains and losses are amortized on a straight-line basis over the expected average remaining service period of active employees in the plan, and over the estimated remaining life expectancy of inactive employees in the plan. Pension plan assets, consisting primarily of listed equity securities as well as corporate and government debt securities, are fair valued at the end of each year. Hydro One records a regulatory asset equal to the net underfunded projected benefit obligation for its pension plan. Post-retirement and Post-employment Benefits Post-retirement and post-employment benefits are recorded and included in rates on an accrual basis. Costs are determined by independent actuaries using the projected benefit method prorated on service and based on assumptions that reflect management’s best estimates. Past service costs from plan amendments are amortized to results of operations based on the expected average remaining service period. For post-retirement benefits, all actuarial gains or losses are deferred using the “corridor” approach. The amount calculated above the “corridor” is amortized to results of operations on a straight-line basis over the expected average remaining service life of active employees in the plan and over the remaining life expectancy of inactive employees in the plan. The post-retirement benefit obligation is remeasured to its fair value at each year end based on an annual actuarial report, with an offset to the associated regulatory asset, to the extent of the remeasurement adjustment. For post-employment obligations, the associated regulatory liabilities representing actuarial gains on transition to US GAAP are amortized to results of operations based on the “corridor” approach. The actuarial gains and losses on post-employment obligations that are incurred during the year are recognized immediately to results of operations. The post-employment benefit obligation is remeasured to its fair value at each year end based on an annual actuarial report, with an offset to the associated regulatory asset, to the extent of the remeasurement adjustment. All post-retirement and post-employment future benefit costs are attributed to labour and are either charged to results of operations or capitalized as part of the cost of property, plant and equipment and intangible assets. Stock-Based Compensation Share Grant Plans Hydro One measures share grant plans based on fair value of share grants as estimated based on the grant date Hydro One Limited’s share price. The costs are recognized in the financial statements using the graded-vesting attribution method for share grant plans that have both a performance condition and a service condition. The Company records a regulatory asset equal to the accrued costs of share grant plans recognized in each period. Forfeitures are recognized as they occur (see note 3). Directors’ Deferred Share Unit (DSU) Plan The Company records the liabilities associated with its Directors’ DSU Plan at fair value at each reporting date until settlement, recognizing compensation expense over the vesting period on a straight-line basis. The fair value of the DSU liability is based on Hydro One Limited’s common share closing price at the end of each reporting period. Long-term Incentive Plan (LTIP) The Company measures its LTIP at fair value based on the grant date share price of Hydro One Limited’s common shares. The related compensation expense of Hydro One is recognized over the vesting period on a straight-line basis. Forfeitures are recognized as they occur. Loss Contingencies Hydro One is involved in certain legal and environmental matters that arise in the normal course of business. In the preparation of its Consolidated Financial Statements, management makes judgments regarding the future outcome of contingent events and records a loss for a contingency based on its best estimate when it is determined that such loss is probable and the amount of the loss can be reasonably estimated. Where the loss amount is recoverable in future rates, a regulatory asset is also recorded. When a range estimate for the probable loss exists and no amount within the range is a better estimate than any other amount, the Company records a loss at the minimum amount within the range. Management regularly reviews current information available to determine whether recorded provisions should be adjusted and whether new provisions are required. Estimating probable losses may require analysis of multiple forecasts and scenarios that often depend on judgments about potential actions by third parties, such as federal, provincial and local courts or regulators. Contingent liabilities are often resolved over long periods of time. Amounts recorded in the Consolidated Financial Statements may differ from the actual outcome once the contingency is resolved. Such difference

New Accounting Pronouncements

New Accounting Pronouncements	12 Months Ended
	Dec. 31, 2016
Accounting Changes and Error Corrections [Abstract]
New Accounting Pronouncements	3. NEW ACCOUNTING PRONOUNCEMENTS The following tables present Accounting Standards Updates (ASUs) issued by the Financial Accounting Standards Board (FASB) that are applicable to Hydro One: Recently Adopted Accounting Guidance ASU Date issued Description Effective date Impact on Hydro One 2014-16 November 2014 This update clarifies that all relevant terms and features should be considered in evaluating the nature of a host contract for hybrid financial instruments issued in the form of a share. The nature of the host contract depends upon the economic characteristics and risks of the entire hybrid financial instrument. January 1, 2016 No material impact upon adoption 2015-01 January 2015 Extraordinary items are no longer required to be presented separately in the income statement. January 1, 2016 No material impact upon adoption 2015-02 February 2015 Guidance on analysis to be performed to determine whether certain types of legal entities should be consolidated. January 1, 2016 No material impact upon adoption 2015-03 April 2015 Debt issuance costs are required to be presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability consistent with debt discounts or premiums. January 1, 2016 Reclassification of deferred debt issuance costs and net unamortized debt premiums as an offset to long-term debt. Applied retrospectively (see note 15). 2015-05 April 2015 Cloud computing arrangements that have been assessed to contain a software licence should be accounted for as internal-use software. January 1, 2016 No material impact upon adoption 2015-16 September 2015 Adjustments to provisional amounts that are identified during the measurement period of a business combination in the reporting period in which the adjustment amount is determined are required to be recognized. The amount recorded in current period earnings are required to be presented separately on the face of the income statement or disclosed in the notes by line item. January 1, 2016 No material impact upon adoption 2015-17 November 2015 All deferred tax assets and liabilities are required to be classified as noncurrent on the balance sheet. January 1, 2017 This ASU was early adopted as of April 1, 2016 and was applied prospectively. As a result, the current portions of the Company’s deferred income tax assets are reclassified as noncurrent assets on the consolidated Balance Sheet. Prior periods were not retrospectively adjusted (see note 7). 2016-09 March 2016 Several aspects of the accounting for share-based payment transactions were simplified, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. January 1, 2017 This ASU was early adopted as of October 1, 2016 and was applied retrospectively. As a result, the Company accounts for forfeitures as they occur. There were no other material impacts upon adoption. Recently Issued Accounting Guidance Not Yet Adopted ASU Date issued Description Effective date Anticipated impact on Hydro One 2014-09 2015-14 2016-08 2016-10 2016-12 2016-20 May 2014 – December 2016 ASU 2014-09 was issued in May 2014 and provides guidance on revenue recognition relating to the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods and services. ASU 2015-14 deferred the effective date of ASU 2014-09 by one year. Additional ASUs were issued in 2016 that simplify transition and provide clarity on certain aspects of the new standard. January 1, 2018 Hydro One has completed its initial assessment and has identified relevant revenue streams. No quantitative determination has been made as a detailed assessment is now underway and will continue through to the third quarter of 2017, with the end result being a determination of the financial impact of this standard. The Company is on track for implementation of this standard by the effective date. 2016-01 January 2016 This update requires equity investments to be measured at fair value with changes in fair value recognized in net income, and requires enhanced disclosures and presentation of financial assets and liabilities in the financial statements. This ASU also simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment. January 1, 2018 Under assessment 2016-02 February 2016 Lessees are required to recognize the rights and obligations resulting from operating leases as assets (right to use the underlying asset for the term of the lease) and liabilities (obligation to make future lease payments) on the balance sheet. January 1, 2019 An initial assessment is currently underway encompassing a review of all existing leases, which will be followed by a detailed review of relevant contracts. No quantitative determination has been made at this time. The Company is on track for implementation of this standard by the effective date. 2016-05 March 2016 The amendments clarify that a change in the counterparty to a derivative instrument that has been designated as the hedging instrument under Topic 815 does not, in and of itself, require de-designation of that hedging relationship provided that all other hedge accounting criteria continue to be met. January 1, 2018 Under assessment 2016-06 March 2016 Contingent call (put) options that are assessed to accelerate the payment of principal on debt instruments need to meet the criteria of being “clearly and closely related” to their debt hosts. January 1, 2017 No material impact 2016-07 March 2016 The requirement to retroactively adopt the equity method of accounting if an investment qualifies for use of the equity method as a result of an increase in the level of ownership or degree of influence has been eliminated. January 1, 2017 No material impact 2016-11 May 2016 This amendment covers the SEC Staff’s rescinding of certain SEC Staff observer comments that are codified in Topic 605 and Topic 932, effective upon the adoption of Topic 606 and Topic 815, effective to coincide with the effective date of Update 2014-16. January 1, 2019 No material impact 2016-13 June 2016 The amendment provides users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. January 1, 2019 Under assessment 2016-15 August 2016 The amendments provide guidance for eight specific cash flow issues with the objective of reducing the existing diversity in practice. January 1, 2018 Under assessment 2016-16 October 2016 The amendment eliminates the prohibition of recognizing current and deferred income taxes for an intra-entity asset transfer, other than inventory, until the asset has been sold to an outside party. The amendment will permit income tax consequences of such transfers to be recognized when the transfer occurs. January 1, 2018 Under assessment 2016-18 November 2016 The amendment requires that restricted cash or restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning and end-of-period balances in the statement of cash flows. January 1, 2018 Under assessment 2017-01 January 2017 The amendment clarifies the definition of a business and provides additional guidance on evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses.

Business Combinations

Business Combinations	12 Months Ended
	Dec. 31, 2016
Business Combinations [Abstract]
Business Combinations	4. BUSINESS COMBINATIONS Acquisition of Great Lakes Power On October 31, 2016, Hydro One acquired Great Lakes Power, an Ontario regulated electricity transmission business operating along the eastern shore of Lake Superior, north and east of Sault Ste. Marie, Ontario from Brookfield Infrastructure Holdings Inc. The total purchase price for Great Lakes Power was approximately $376 million, including the assumption of approximately $150 million in outstanding indebtedness. The following table summarizes the determination of the final fair value of the assets acquired and liabilities assumed: (millions of dollars) Cash and cash equivalents 5 Property, plant and equipment 221 Intangible assets 1 Regulatory assets 50 Goodwill 159 Working capital (2 ) Long-term debt (186 ) Pension and post-employment benefit liabilities, net (5 ) Deferred income taxes (17 ) 226 Goodwill of approximately $159 million arising from the Great Lakes Power acquisition consists largely of the synergies and economies of scale expected from combining the operations of Hydro One and Great Lakes Power. Great Lakes Power contributed revenues of $6 million and less than $1 million of net income to the Company’s consolidated financial results for the year ended December 31, 2016. All costs related to the acquisition have been expensed through the Consolidated Statements of Operations and Comprehensive Income. Great Lakes Power’s financial information is not material to the Company’s consolidated financial results for the year ended December 31, 2016 and therefore, has not been disclosed on a pro forma basis. On January 16, 2017, the name of Great Lakes Power was changed to Hydro One Sault Ste. Marie LP. Agreement to Purchase Orillia Power On August 15, 2016, the Company reached an agreement to acquire Orillia Power Distribution Corporation (Orillia Power), an electricity distribution company located in Simcoe County, Ontario, from the City of Orillia for approximately $41 million, including the assumption of approximately $15 million in outstanding indebtedness and regulatory liabilities, subject to closing adjustments. The acquisition is subject to regulatory approval by the OEB. Acquisition of Woodstock Hydro On October 31, 2015, Hydro One acquired Woodstock Hydro Holdings Inc. (Woodstock Hydro), an electricity distribution company located in southwestern Ontario. The total purchase price for Woodstock Hydro was approximately $32 million. The purchase price was finalized and the Company made the final purchase price payment of $3 million in 2016. The following table summarizes the determination of the fair value of the assets acquired and liabilities assumed: (millions of dollars) Working capital 4 Property, plant and equipment 27 Intangible assets 1 Deferred income tax assets 2 Goodwill 22 Long-term debt (17 ) Derivative instruments (3 ) Post-retirement and post-employment benefit liability (1 ) Regulatory liabilities (1 ) Other long-term liabilities (2 ) 32 Goodwill of approximately $22 million arising from the Woodstock Hydro acquisition consists largely of the synergies and economies of scale expected from combining the operations of Hydro One and Woodstock Hydro. All of the goodwill was assigned to Hydro One’s Distribution Business segment. Woodstock Hydro contributed revenues of $12 million and net income of $2 million to the Company’s consolidated financial results for the year ended December 31, 2015. All costs related to the acquisition have been expensed through the Consolidated Statements of Operations and Comprehensive Income. Woodstock Hydro’s financial information is not material to the Company’s consolidated financial results for the year ended December 31, 2015 and therefore, has not been disclosed on a pro forma basis. Acquisition of Haldimand Hydro On June 30, 2015, Hydro One acquired Haldimand County Utilities Inc. (Haldimand Hydro), an electricity distribution company located in southwestern Ontario. The total purchase price for Haldimand Hydro was approximately $73 million. The purchase price was finalized in 2016. The following table summarizes the determination of the fair value of the assets acquired and liabilities assumed: (millions of dollars) Cash and cash equivalents 3 Working capital 5 Property, plant and equipment 52 Deferred income tax assets 1 Goodwill 33 Long-term debt (18 ) Regulatory liabilities (3 ) 73 Goodwill of approximately $33 million arising from the Haldimand Hydro acquisition consists largely of the synergies and economies of scale expected from combining the operations of Hydro One and Haldimand Hydro. All of the goodwill was assigned to Hydro One’s Distribution Business segment. Haldimand Hydro contributed revenues of $32 million and net income of $6 million to the Company’s consolidated financial results for the year ended December 31, 2015. All costs related to the acquisition have been expensed through the Consolidated Statements of Operations and Comprehensive Income. Haldimand Hydro’s financial information is not material to the Company’s consolidated financial results for the year ended December 31, 2015 and therefore, has not been disclosed on a pro forma basis. Hydro One Brampton Spin-off On August 31, 2015, Hydro One completed the spin-off of its subsidiary, Hydro One Brampton. The spin-off was accounted as a non-monetary, nonreciprocal transfer with the Province, based on its carrying values at August 31, 2015. Transactions that immediately preceded the spin-off as well as the spin-off were as follows: • Hydro One subscribed for 357 common shares of Hydro One Brampton for an aggregate subscription price of $53 million; and • Hydro One transferred to a company wholly owned by the Province all the issued and outstanding shares of Hydro One Brampton as a dividend-in-kind; and all of the long-term intercompany debt in aggregate principal amount of $193 million plus accrued interest of $3 million owed by Hydro One Brampton to Hydro One as a return of stated capital of $196 million on its common shares. As a result of the spin-off, goodwill related to Hydro One Brampton of $60 million was eliminated from the Consolidated Balance Sheet. Other On November 6, 2015, Hydro One completed the spin-off of its subsidiary, Hydro One Telecom. The spin-off was accounted as a non-monetary, nonreciprocal transfer with Hydro One Limited, based on its carrying values at November 6, 2015. Hydro One transferred to Hydro One Limited all the issued and outstanding shares of Hydro One Telecom totalling $17 million, and all of the intercompany debt receivable held by Hydro One due from Hydro One Telecom and Hydro One Telecom Link Limited totalling $21 million, as a return of stated capital of $38 million on its common shares. On November 6, 2015, Hydro One completed the spin-off of its subsidiary, Municipal Billing Services Inc. (MBSI). The spin-off was accounted as a non-monetary, nonreciprocal transfer with Hydro One Limited, based on its carrying values at November 6, 2015. Hydro One transferred to Hydro One Limited all the issued and outstanding shares of MBSI and all of the intercompany debt receivable held by Hydro One due from MBSI, as a return of stated capital of $3 million on its common shares.

Depreciation and Amortization

Depreciation and Amortization	12 Months Ended
	Dec. 31, 2016
Text Block [Abstract]
Depreciation and Amortization	5. DEPRECIATION AND AMORTIZATION Year ended December 31 (millions of dollars) 2016 2015 Depreciation of property, plant and equipment 603 594 Asset removal costs 90 90 Amortization of intangible assets 56 54 Amortization of regulatory assets 20 19 769 757

Financing Charges

Financing Charges	12 Months Ended
	Dec. 31, 2016
Banking and Thrift, Interest [Abstract]
Financing Charges	6. FINANCING CHARGES Year ended December 31 (millions of dollars) 2016 2015 Interest on long-term debt 424 417 Interest on short-term notes 9 2 Other 15 14 Less: Interest capitalized on construction and development in progress (54 ) (52 ) Interest earned on investments (2 ) (3 ) Gain on interest-rate swap agreements — (2 ) 392 376

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income Taxes	7. INCOME TAXES Income taxes / provision for PILs differs from the amount that would have been recorded using the combined Canadian federal and Ontario statutory income tax rate. The reconciliation between the statutory and the effective tax rates is provided as follows: Year ended December 31 (millions of dollars) 2016 2015 Income taxes / provision for PILs at statutory rate 231 216 Increase (decrease) resulting from: Net temporary differences recoverable in future rates charged to customers: Capital cost allowance in excess of depreciation and amortization (53 ) (37 ) Pension contributions in excess of pension expense (16 ) (25 ) Overheads capitalized for accounting but deducted for tax purposes (16 ) (15 ) Interest capitalized for accounting but deducted for tax purposes (14 ) (13 ) Environmental expenditures (5 ) (5 ) Other 5 (6 ) Net temporary differences (99 ) (101 ) Net tax benefit resulting from transition from PILs Regime to Federal Tax Regime — (9 ) Hydro One Brampton spin-off — 7 Net permanent differences 3 1 Total income taxes / provision for PILs 135 114 The major components of income tax expense are as follows: Year ended December 31 (millions of dollars) 2016 2015 Current income taxes / provision for PILs 24 2,931 Deferred income taxes / provision for (recovery of) PILs 111 (2,817 ) Total income taxes / provision for PILs 135 114 Effective income tax rate 15.5 % 14.0 % The provision for current income taxes / PILs is remitted to the CRA (Federal Tax Regime) and the OEFC (PILs Regime). At December 31, 2016, $13 million (2015 – $1 million) receivable from the CRA was included in other current assets and $6 million (2015 – $12 million) receivable from the OEFC was included in due from related parties on the Consolidated Balance Sheet. In connection with Hydro One Limited’s IPO in 2015, Hydro One’s exemption from tax under the Federal Tax Regime ceased to apply. Under the PILs Regime, Hydro One was deemed to have disposed of its assets immediately before it lost its tax exempt status under the Federal Tax Regime, resulting in Hydro One making payments in lieu of tax (Departure Tax) totalling $2.6 billion. To enable Hydro One to make the Departure Tax payment, Hydro One Limited subscribed for common shares of Hydro One for $2.6 billion in 2015 (see note 21). Hydro One used the proceeds of this share subscription to pay the Departure Tax. The 2015 total income taxes / provision for PILs included a current provision of $2,600 million and a deferred recovery of $2,798 million resulting from the transition from the PILs Regime to the Federal Tax Regime. The deferred recovery was not included in the rate-setting process. Deferred income tax balances expected to be included in the rate-setting process are offset by regulatory assets and liabilities to reflect the anticipated recovery or disposition of these balances within future electricity rates. Deferred Income Tax Assets and Liabilities Deferred income tax assets and liabilities arise from differences between the carrying amounts and tax basis of the Company’s assets and liabilities. At December 31, 2016 and 2015, deferred income tax assets and liabilities consisted of the following: December 31 (millions of dollars) 2016 2015 Deferred income tax assets Depreciation and amortization in excess of capital cost allowance 477 918 Non-depreciable capital property 271 271 Post-retirement and post-employment benefits expense in excess of cash payments 603 572 Environmental expenditures 74 75 Non-capital losses 213 62 Investment in subsidiaries 75 55 Other 30 2 1,743 1,955 Less: valuation allowance (352 ) (326 ) Total deferred income tax assets 1,391 1,629 Less: current portion — 19 1,391 1,610 December 31 (millions of dollars) 2016 2015 Deferred income tax liabilities Regulatory amounts that are not recognized for tax purposes (153 ) (153 ) Goodwill (10 ) (10 ) Capital cost allowance in excess of depreciation and amortization (64 ) (42 ) Other (11 ) (1 ) Total deferred income tax liabilities (238 ) (206 ) Less: current portion — — (238 ) (206 ) Net deferred income tax assets 1,153 1,423 The net deferred income tax assets are presented on the Consolidated Balance Sheets as follows: December 31 (millions of dollars) 2016 2015 Current: Other current assets — 19 Long-term: Deferred income tax assets 1,213 1,610 Deferred income tax liabilities (60 ) (206 ) Net deferred income tax assets 1,153 1,423 The valuation allowance for deferred tax assets as at December 31, 2016 was $352 million (2015 – $326 million). The valuation allowance primarily relates to temporary differences for non-depreciable assets and investments in subsidiaries. As of December 31, 2016, the Company had non-capital losses carried forward available to reduce future years’ taxable income, which expire as follows: Year of expiry (millions of dollars) 2016 2015 2034 2 2 2035 221 232 2036 579 — Total losses 802 234

Accounts Receivable

Accounts Receivable	12 Months Ended
	Dec. 31, 2016
Receivables [Abstract]
Accounts Receivable	8. ACCOUNTS RECEIVABLE December 31 (millions of dollars) 2016 2015 Accounts receivable – billed 427 374 Accounts receivable – unbilled 441 459 Accounts receivable, gross 868 833 Allowance for doubtful accounts (35 ) (61 ) Accounts receivable, net 833 772 The following table shows the movements in the allowance for doubtful accounts for the years ended December 31, 2016 and 2015: Year ended December 31 (millions of dollars) 2016 2015 Allowance for doubtful accounts – January 1 (61 ) (66 ) Write-offs 37 37 Additions to allowance for doubtful accounts (11 ) (32 ) Allowance for doubtful accounts – December 31 (35 ) (61 )

Other Current Assets

Other Current Assets	12 Months Ended
	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Other Current Assets	9. OTHER CURRENT ASSETS December 31 (millions of dollars) 2016 2015 Regulatory assets (Note 12) 37 36 Materials and supplies 19 21 Deferred income tax assets (Notes 3, 7) — 19 Prepaid expenses and other assets 41 24 97 100

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	10. PROPERTY, PLANT AND EQUIPMENT December 31, 2016 (millions of dollars) Property, Plant and Accumulated Construction in Progress Total Transmission 14,692 4,862 910 10,740 Distribution 9,656 3,305 243 6,594 Communication 1,069 674 9 404 Administration and service 1,632 924 61 769 Easements 628 67 — 561 27,677 9,832 1,223 19,068 December 31, 2015 (millions of dollars) Property, Plant and Accumulated Construction in Progress Total Transmission 13,704 4,621 853 9,936 Distribution 9,205 3,177 238 6,266 Communication 1,006 609 18 415 Administration and service 1,531 848 35 718 Easements 622 64 — 558 26,068 9,319 1,144 17,893 Financing charges capitalized on property, plant and equipment under construction were $52 million in 2016 (2015 – $50 million).

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets	11. INTANGIBLE ASSETS December 31, 2016 (millions of dollars) Intangible Assets Accumulated Development in Progress Total Computer applications software 621 326 53 348 Other 5 4 — 1 626 330 53 349 December 31, 2015 (millions of dollars) Intangible Assets Accumulated Development in Progress Total Computer applications software 579 270 24 333 Other 7 4 — 3 586 274 24 336 Financing charges capitalized to intangible assets under development were $2 million in 2016 (2015 – $1 million). The estimated annual amortization expense for intangible assets is as follows: 2017 – $54 million; 2018 – $54 million; 2019 – $45 million; 2020 – $27 million; and 2021 – $26 million.

Regulatory Assets and Liabiliti

Regulatory Assets and Liabilities	12 Months Ended
	Dec. 31, 2016
Regulated Operations [Abstract]
Regulatory Assets and Liabilities	12. REGULATORY ASSETS AND LIABILITIES Regulatory assets and liabilities arise as a result of the rate-setting process. Hydro One has recorded the following regulatory assets and liabilities: December 31 (millions of dollars) 2016 2015 Regulatory assets: Deferred income tax regulatory asset 1,587 1,445 Pension benefit regulatory asset 900 952 Post-retirement and post-employment benefits 243 240 Environmental 204 207 Retail settlement variance account 145 110 Debt premium 32 — Share-based compensation 31 10 Distribution system code exemption 10 10 2015-2017 rate rider 7 20 B2M LP start-up costs 5 8 Pension cost variance 4 37 Other 14 12 Total regulatory assets 3,182 3,051 Less: current portion 37 36 3,145 3,015 Regulatory liabilities: Green Energy expenditure variance 69 76 External revenue variance 64 87 CDM deferral variance 54 53 Deferred income tax regulatory liability 4 23 Other 18 16 Total regulatory liabilities 209 255 Less: current portion — 19 209 236 Deferred Income Tax Regulatory Asset and Liability Deferred income taxes are recognized on temporary differences between the carrying amount of assets and liabilities in the financial statements and the corresponding tax bases used in the computation of taxable income. The Company has recognized regulatory assets and liabilities that correspond to deferred income taxes that flow through the rate-setting process. In the absence of rate-regulated accounting, the Company’s income tax expense would have been recognized using the liability method and there would be no regulatory accounts established for taxes to be recovered through future rates. As a result, the 2016 income tax expense would have been higher by approximately $104 million (2015 – $101 million). Pension Benefit Regulatory Asset In accordance with OEB rate orders, pension costs are recovered on a cash basis as employer contributions are paid to the pension fund in accordance with the Pension Benefits Act Post-Retirement and Post-Employment Benefits The Company recognizes the net unfunded status of post-retirement and post-employment obligations on the Consolidated Balance Sheets with an incremental offset to the associated regulatory assets. A regulatory asset is recognized because management considers it to be probable that post-retirement and post-employment benefit costs will be recovered in the future through the rate-setting process. The post-retirement and post-employment benefit obligation is remeasured to its fair value at each year end based on an annual actuarial report, with an offset to the associated regulatory asset, to the extent of the re-measurement adjustment. In the absence of rate-regulated accounting, 2016 OCI would have been lower by $3 million (2015 – higher by $33 million). Environmental Hydro One records a liability for the estimated future expenditures required to remediate environmental contamination. Because such expenditures are expected to be recoverable in future rates, the Company has recorded an equivalent amount as a regulatory asset. In 2016, the environmental regulatory asset decreased by $1 million (2015 – $24 million) to reflect related changes in the Company’s PCB liability, and increased by $10 million (2015 – $1 million) due to changes in the land assessment and remediation liability. The environmental regulatory asset is amortized to results of operations based on the pattern of actual expenditures incurred and charged to environmental liabilities. The OEB has the discretion to examine and assess the prudency and the timing of recovery of all of Hydro One’s actual environmental expenditures. In the absence of rate-regulated accounting, 2016 operation, maintenance and administration expenses would have been higher by $9 million (2015 – lower by $23 million). In addition, 2016 amortization expense would have been lower by $20 million (2015 – $19 million), and 2016 financing charges would have been higher by $8 million (2015 – $10 million). Retail Settlement Variance Account (RSVA) Hydro One has deferred certain retail settlement variance amounts under the provisions of Article 490 of the OEB’s Accounting Procedures Handbook. In March 2015, the OEB approved the disposition of the total RSVA balance accumulated from January 2012 to December 2013, including accrued interest, to be recovered through the 2015-2017 Rate Rider. Debt Premium The value of debt assumed in the acquisition of Great Lakes Power has been recorded at fair value in accordance with US GAAP – Business Combinations. The OEB allows for recovery of interest at the coupon rate of the Senior Secured Bonds and a regulatory asset has been recorded for the difference between the fair value and face value of this debt. The debt premium is recovered over the remaining term of the debt (see note 15). Share-based Compensation The Company recognizes costs associated with share grant plans in a regulatory asset as management considers it probable that share grant plans costs will be recovered in the future through the rate-setting process. In the absence of rate-regulated accounting, 2016 operation, maintenance and administration expenses would have been higher by $9 million (2015 – $5 million). Distribution System Code (DSC) Exemption In June 2010, Hydro One Networks filed an application with the OEB regarding the OEB’s new cost responsibility rules contained in the OEB’s October 2009 Notice of Amendment to the DSC, with respect to the connection of certain renewable generators that were already connected or that had received a connection impact assessment prior to October 21, 2009. The application sought approval to record and defer the unanticipated costs incurred by Hydro One Networks that resulted from the connection of certain renewable generation facilities. The OEB ruled that identified specific expenditures can be recorded in a deferral account subject to the OEB’s review in subsequent Hydro One Network distribution applications. In March 2015, the OEB approved the disposition of the DSC exemption deferral account at December 31, 2013, including accrued interest, which is being recovered through the 2015-2017 2015-2017 Rate Rider In March 2015, as part of its decision on Hydro One Networks’ distribution rate application for 2015-2019, the OEB approved the disposition of certain deferral and variance accounts, including RSVAs and accrued interest. The 2015-2017 Rate Rider account includes the balances approved for disposition by the OEB and is being disposed in accordance with the OEB decision over a 32-month B2M LP Start-up Costs In December 2015, OEB issued its decision on B2M LP’s application for 2015-2019 Pension Cost Variance A pension cost variance account was established for Hydro One Networks’ transmission and distribution businesses to track the difference between the actual pension expenses incurred and estimated pension costs approved by the OEB. The balance in this regulatory account reflects the excess of pension costs paid as compared to OEB-approved amounts. In March 2015, the OEB approved the disposition of the distribution business portion of the total pension cost variance account at December 31, 2013, including accrued interest, which is being recovered through the 2015-2017 Rate Rider. In the absence of rate-regulated accounting, 2016 revenue would have been higher by $25 million (2015 – lower by $6 million). Green Energy Expenditure Variance In April 2010, the OEB requested the establishment of deferral accounts which capture the difference between the revenue recorded on the basis of Green Energy Plan expenditures incurred and the actual recoveries received. External Revenue Variance In May 2009, the OEB approved forecasted amounts related to export service revenue, external revenue from secondary land use, and external revenue from station maintenance and engineering and construction work. In November 2012, the OEB again approved forecasted amounts related to these revenue categories and extended the scope to encompass all other external revenues. The external revenue variance account balance reflects the excess of actual external revenues compared to the OEB-approved forecasted amounts. CDM Deferral Variance Account As part of Hydro One Networks’ application for 2013 and 2014 transmission rates, Hydro One agreed to establish a new regulatory deferral variance account to track the impact of actual Conservation and Demand Management (CDM) and demand response results on the load forecast compared to the estimated load forecast included in the revenue requirement. The balance in the CDM deferral variance account relates to the actual 2013 and 2014 CDM compared to the amounts included in 2013 and 2014 revenue requirements, respectively. There were no additions to this regulatory account in 2016.

Accounts Payable and Other Curr

Accounts Payable and Other Current Liabilities	12 Months Ended
	Dec. 31, 2016
Payables and Accruals [Abstract]
Accounts Payable and Other Current Liabilities	13. ACCOUNTS PAYABLE AND OTHER CURRENT LIABILITIES December 31 (millions of dollars) 2016 2015 Accounts payable 177 152 Accrued liabilities 651 591 Accrued interest 105 96 Regulatory liabilities (Note 12) — 19 933 858

Other Long-Term Liabilities

Other Long-Term Liabilities	12 Months Ended
	Dec. 31, 2016
Other Liabilities Disclosure [Abstract]
Other Long-Term Liabilities	14. OTHER LONG-TERM LIABILITIES December 31 (millions of dollars) 2016 2015 Post-retirement and post-employment benefit liability (Note 18) 1,628 1,541 Pension benefit liability (Note 18) 900 952 Environmental liabilities (Note 19) 177 185 Due to related parties (Note 26) 26 10 Asset retirement obligations (Note 20) 9 9 Long-term accounts payable and other liabilities 25 17 2,765 2,714

Debt and Credit Agreements

Debt and Credit Agreements	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Debt and Credit Agreements	15. DEBT AND CREDIT AGREEMENTS Short-Term Notes and Credit Facilities Hydro One meets its short-term liquidity requirements in part through the issuance of commercial paper under its Commercial Paper Program which has a maximum authorized amount of $1.5 billion. These short-term notes are denominated in Canadian dollars with varying maturities up to 365 days. The Commercial Paper Program is supported by the Company’s committed revolving credit facilities totalling $2.3 billion. On August 15, 2016, Hydro One terminated its $1.5 billion revolving standby credit facility maturing in June 2020 and its $800 million three-year senior, revolving term credit facility maturing in October 2018 (collectively, Prior Credit Facilities). On the same date, Hydro One entered into a new credit agreement for a $2.3 billion revolving credit facility maturing in June 2021 (New Credit Facility). The New Credit Facility ranks equally with any existing and future senior debt of Hydro One, and has customary covenants substantially similar to the covenants under the Prior Credit Facilities. The Company may use the credit facilities for working capital and general corporate purposes. If used, interest on the credit facilities would apply based on Canadian benchmark rates. The obligation of each lender to make any credit extension under its credit facility is subject to various conditions including that no event of default has occurred or would result from such credit extension. Long-Term Debt At December 31, 2016, $10,523 million long-term debt was issued by the Company under its Medium-Term Note (MTN) Program. The maximum authorized principal amount of notes issuable under the current MTN Program prospectus filed in December 2015 is $3.5 billion. At December 31, 2016, $1.2 billion remained available for issuance until January 2018. In addition, at December 31, 2016, the Company had long-term debt of $184 million assumed as part of the Great Lakes Power acquisition. The following table presents outstanding long-term debt at December 31, 2016 and 2015: December 31 (millions of dollars) 2016 2015 4.64% Series 10 notes due 2016 — 450 Floating-rate Series 27 notes due 2016 1 — 50 5.18% Series 13 notes due 2017 600 600 2.78% Series 28 notes due 2018 750 750 Floating-rate Series 31 notes due 2019 1 228 228 1.48% Series 37 notes due 2019 2 500 — 4.40% Series 20 notes due 2020 300 300 1.62% Series 33 notes due 2020 2 350 350 1.84% Series 34 notes due 2021 500 — 3.20% Series 25 notes due 2022 600 600 2.77% Series 35 notes due 2026 500 — 7.35% Debentures due 2030 400 400 6.93% Series 2 notes due 2032 500 500 6.35% Series 4 notes due 2034 385 385 5.36% Series 9 notes due 2036 600 600 4.89% Series 12 notes due 2037 400 400 6.03% Series 17 notes due 2039 300 300 5.49% Series 18 notes due 2040 500 500 4.39% Series 23 notes due 2041 300 300 6.59% Series 5 notes due 2043 315 315 4.59% Series 29 notes due 2043 435 435 4.17% Series 32 notes due 2044 350 350 5.00% Series 11 notes due 2046 325 325 3.91% Series 36 notes due 2046 350 — 3.72% Series 38 notes due 2047 450 — 4.00% Series 24 notes due 2051 225 225 3.79% Series 26 notes due 2062 310 310 4.29% Series 30 notes due 2064 50 50 Hydro One Inc. long-term debt 10,523 8,723 6.6% Senior Secured Bonds due 2023 (Face value – $112 million) 144 — 4.6% Note Payable due 2023 (Face value – $36 million) 40 — Great Lakes Power long-term debt 184 — 10,707 8,723 Add: Net unamortized debt premiums 3 15 17 Add: Unrealized mark-to-market loss (gain) 2 (2 ) 1 Less: Deferred debt issuance costs 3 (40 ) (34 ) Total long-term debt 10,680 8,707 1 The interest rates of the floating-rate notes are referenced to the 3-month Canadian dollar bankers’ acceptance rate, plus a margin. 2 The unrealized mark-to-market net gain relates to $50 million of the Series 33 notes due 2020 and $500 million Series 37 notes due 2019 (2015 – loss relates to $50 million of the Series 33 notes due 2020). The unrealized mark-to-market net gain is offset by a $2 million (2015 – $1 million) unrealized mark-to-market net loss (2015 – gain) on the related fixed-to-floating interest-rate swap agreements, which are accounted for as fair value hedges. See note 16 – Fair Value of Financial Instruments and Risk Management for details of fair value hedges. 3 Effective January 1, 2016, deferred debt issuance costs and net unamortized debt premiums were reclassified from other long-term assets and other long-term liabilities, respectively, as an offset to long-term debt upon adoption of ASU 2015-03 The total long-term debt is presented on the consolidated balance sheets as follows: December 31 (millions of dollars) 2016 2015 Current liabilities: Long-term debt payable within one year 602 500 Long-term liabilities: Long-term debt 10,078 8,207 Total long-term debt 10,680 8,707 In 2016, Hydro One issued $2,300 million (2015 – $350 million) of long-term debt under the MTN Program, and repaid $502 million (2015 – $550 million) of total long-term debt. Principal repayments and related weighted average interest rates are summarized by the number of years to maturity in the following table: Long-term Debt Principal Repayments Weighted Average Interest Rate Years to Maturity (millions of dollars) (%) 1 year 602 5.2 2 years 753 2.8 3 years 731 1.4 4 years 653 2.9 5 years 503 1.9 3,242 2.8 6 – 10 years 1,234 3.3 Over 10 years 6,195 5.2 10,671 4.3 Interest payment obligations related to long-term debt are summarized by year in the following table: Interest Payments Year (millions of dollars) 2017 456 2018 425 2019 402 2020 384 2021 370 2,037 2022-2026 1,703 2027+ 4,405 8,145

Fair Value of Financial Instrum

Fair Value of Financial Instruments and Risk Management	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments and Risk Management	16. FAIR VALUE OF FINANCIAL INSTRUMENTS AND RISK MANAGEMENT Fair value is considered to be the exchange price in an orderly transaction between market participants to sell an asset or transfer a liability at the measurement date. The fair value definition focuses on an exit price, which is the price that would be received in the sale of an asset or the amount that would be paid to transfer a liability. Hydro One classifies its fair value measurements based on the following hierarchy, as prescribed by the accounting guidance for fair value, which prioritizes the inputs to valuation techniques used to measure fair value into three levels: Level 1 inputs are unadjusted quoted prices in active markets for identical assets or liabilities that Hydro One has the ability to access. An active market for the asset or liability is one in which transactions for the asset or liability occur with sufficient frequency and volume to provide ongoing pricing information. Level 2 inputs are those other than quoted market prices that are observable, either directly or indirectly, for an asset or liability. Level 2 inputs include, but are not limited to, quoted prices for similar assets or liabilities in an active market, quoted prices for identical or similar assets or liabilities in markets that are not active and inputs other than quoted market prices that are observable for the asset or liability, such as interest-rate curves and yield curves observable at commonly quoted intervals, volatilities, credit risk and default rates. A Level 2 measurement cannot have more than an insignificant portion of the valuation based on unobservable inputs. Level 3 inputs are any fair value measurements that include unobservable inputs for the asset or liability for more than an insignificant portion of the valuation. A Level 3 measurement may be based primarily on Level 2 inputs. Non-Derivative Financial Assets and Liabilities At December 31, 2016 and 2015, the Company’s carrying amounts of cash and cash equivalents, accounts receivable, due from related parties, short-term notes payable, accounts payable, and due to related parties are representative of fair value because of the short-term nature of these instruments. Fair Value Measurements of Long-Term Debt The fair values and carrying values of the Company’s long-term debt at December 31, 2016 and 2015 are as follows: December 31 (millions of dollars) 2016 Carrying Value 2016 Fair Value 2015 Carrying Value 2015 Fair Value Long-term debt $50 million of MTN Series 33 notes 50 50 51 51 $500 million of MTN Series 37 notes 498 498 — — Other notes and debentures 10,132 11,462 8,656 9,942 10,680 12,010 8,707 9,993 Fair Value Measurements of Derivative Instruments At December 31, 2016, Hydro One had interest-rate swaps in the amount of $550 million (2015 – $50 million) that was used to convert fixed-rate debt to floating-rate debt. These swaps are classified as a fair value hedges. Hydro One’s fair value hedge exposure was equal to about 5% (2015 – 1%) of its total long-term debt. At December 31, 2016, Hydro One had the following interest-rate swaps designated as fair value hedges: • a $50 million fixed-to-floating interest-rate swap agreement to convert $50 million of the $350 million MTN Series 33 notes maturing April 30, 2020 into three-month variable rate debt; and • two $125 million and one $250 million fixed-to-floating interest-rate swap agreements to convert the $500 million MTN Series 37 notes maturing November 18, 2019 into three-month variable rate debt. At December 31, 2016 and 2015, the Company had no interest-rate swaps classified as undesignated contracts. Fair Value Hierarchy The fair value hierarchy of financial assets and liabilities at December 31, 2016 and 2015 is as follows: December 31, 2016 (millions of dollars) Carrying Value Fair Value Level 1 Level 2 Level 3 Assets: Cash and cash equivalents 48 48 48 — — 48 48 48 — — Liabilities: Short-term notes payable 469 469 469 — — Long-term debt, including current portion 10,680 12,010 — 12,010 — Derivative instruments Fair value hedges – interest-rate swaps 2 2 2 — — 11,151 12,481 471 12,010 — December 31, 2015 (millions of dollars) Carrying Value Fair Value Level 1 Level 2 Level 3 Assets: Cash and cash equivalents 89 89 89 — — Derivative instruments Fair value hedge – interest-rate swap 1 1 1 — — 90 90 90 — — Liabilities: Short-term notes payable 1,491 1,491 1,491 — — Long-term debt, including current portion 8,707 9,993 — 9,993 — 10,198 11,484 1,491 9,993 — Cash and cash equivalents include cash and short-term investments. The carrying values are representative of fair value because of the short-term nature of these instruments. The fair value of the hedged portion of the long-term debt is primarily based on the present value of future cash flows using a swap yield curve to determine the assumption for interest rates. The fair value of the unhedged portion of the long-term debt is based on unadjusted period-end market prices for the same or similar debt of the same remaining maturities. There were no significant transfers between any of the fair value levels during the years ended December 31, 2016 and 2015. Risk Management Exposure to market risk, credit risk and liquidity risk arises in the normal course of the Company’s business. Market Risk Market risk refers primarily to the risk of loss that results from changes in costs, foreign exchange rates and interest rates. The Company is exposed to fluctuations in interest rates as its regulated return on equity is derived using a formulaic approach that takes into account anticipated interest rates. The Company is not currently exposed to material commodity price risk or material foreign exchange risk. The Company uses a combination of fixed and variable-rate debt to manage the mix of its debt portfolio. The Company also uses derivative financial instruments to manage interest-rate risk. The Company utilizes interest-rate swaps, which are typically designated as fair value hedges, as a means to manage its interest rate exposure to achieve a lower cost of debt. The Company may also utilize interest-rate derivative instruments to lock in interest-rate levels in anticipation of future financing. A hypothetical 100 basis points increase in interest rates associated with variable-rate debt would not have resulted in a significant decrease in Hydro One’s net income for the years ended December 31, 2016 or 2015. For derivative instruments that are designated and qualify as fair value hedges, the gain or loss on the derivative instrument as well as the offsetting loss or gain on the hedged item attributable to the hedged risk are recognized in the Consolidated Statements of Operations and Comprehensive Income. The net unrealized loss (gain) on the hedged debt and the related interest-rate swaps for the years ended December 31, 2016 and 2015 was not significant. Credit Risk Financial assets create a risk that a counterparty will fail to discharge an obligation, causing a financial loss. At December 31, 2016 and 2015, there were no significant concentrations of credit risk with respect to any class of financial assets. The Company’s revenue is earned from a broad base of customers. As a result, Hydro One did not earn a significant amount of revenue from any single customer. At December 31, 2016 and 2015, there was no significant accounts receivable balance due from any single customer. At December 31, 2016, the Company’s provision for bad debts was $35 million (2015 – $61 million). Adjustments and write-offs were determined on the basis of a review of overdue accounts, taking into consideration historical experience. At December 31, 2016, approximately 6% (2015 – 6%) of the Company’s net accounts receivable were aged more than 60 days. Hydro One manages its counterparty credit risk through various techniques including: entering into transactions with highly rated counterparties; limiting total exposure levels with individual counterparties; entering into master agreements which enable net settlement and the contractual right of offset; and monitoring the financial condition of counterparties. The Company monitors current credit exposure to counterparties both on an individual and an aggregate basis. The Company’s credit risk for accounts receivable is limited to the carrying amounts on the Consolidated Balance Sheets. Derivative financial instruments result in exposure to credit risk since there is a risk of counterparty default. The credit exposure of derivative contracts, before collateral, is represented by the fair value of contracts at the reporting date. At December 31, 2016 and 2015, the counterparty credit risk exposure on the fair value of these interest-rate swap contracts was not significant. At December 31, 2016, Hydro One’s credit exposure for all derivative instruments, and applicable payables and receivables, had a credit rating of investment grade, with four financial institutions as the counterparty. Liquidity Risk Liquidity risk refers to the Company’s ability to meet its financial obligations as they come due. Hydro One meets its short-term liquidity requirements using cash and cash equivalents on hand, funds from operations, the issuance of commercial paper, and the revolving standby credit facilities. The short-term liquidity under the Commercial Paper Program, revolving standby credit facilities, and anticipated levels of funds from operations are expected to be sufficient to fund normal operating requirements. At December 31, 2016, accounts payable and accrued liabilities in the amount of $828 million (2015 – $743 million) were expected to be settled in cash at their carrying amounts within the next 12 months.

Capital Management

Capital Management	12 Months Ended
	Dec. 31, 2016
Text Block [Abstract]
Capital Management	17. CAPITAL MANAGEMENT The Company’s objectives with respect to its capital structure are to maintain effective access to capital on a long-term basis at reasonable rates, and to deliver appropriate financial returns. In order to ensure ongoing access to capital, the Company targets to maintain strong credit quality. At December 31, 2016 and 2015, the Company’s capital structure was as follows: December 31 (millions of dollars) 2016 2015 Long-term debt payable within one year 602 500 Short-term notes payable 469 1,491 Less: cash and cash equivalents 48 89 1,023 1,902 Long-term debt 10,078 8,207 Common shares 5,391 6,000 Retained earnings 4,487 3,759 Total capital 20,979 19,868 Hydro One and Great Lakes Power have customary covenants typically associated with long-term debt. Hydro One’s long-term debt and credit facility covenants limit permissible debt to 75% of its total capitalization, limit the ability to sell assets and impose a negative pledge provision, subject to customary exceptions. At December 31, 2016, Hydro One and Great Lakes Power were in compliance with all covenants and limitations.

Pension and Post-Retirement and

Pension and Post-Retirement and Post-Employment Benefits	12 Months Ended
	Dec. 31, 2016
Postemployment Benefits [Abstract]
Pension and Post-Retirement and Post-Employment Benefits	18. PENSION AND POST-RETIREMENT AND POST-EMPLOYMENT BENEFITS Hydro One has a defined benefit pension plan (Pension Plan), a defined contribution pension plan (DC Plan), a supplementary pension plan, and post-retirement and post-employment benefit plans. Defined Contribution Pension Plan Hydro One established a DC Plan effective January 1, 2016. The DC Plan is mandatory and covers eligible management employees hired on or after January 1, 2016, as well as management employees hired before January 1, 2016 who were not eligible or had not irrevocably elected to join the Pension Plan as of September 30, 2015. Members of the DC Plan have an option to contribute 4%, 5% or 6% of their pensionable earnings, with matching contributions by Hydro One. Hydro One contributions to the DC Plan for the year ended December 31, 2016 were less than $1 million (2015 – $nil). At December 31, 2016, Company contributions payable included in accrued liabilities on the Consolidated Balance Sheets were less than $1 million (2015 – $nil). Defined Benefit Pension Plan, Supplementary Pension Plan, and Post-Retirement and Post-Employment Plans The Pension Plan is a defined benefit contributory plan which covers all regular employees of Hydro One and its subsidiaries. The Pension Plan provides benefits based on highest three-year average pensionable earnings. For Management employees who commenced employment on or after January 1, 2004, and for Society of Energy Professionals-represented staff hired after November 17, 2005, benefits are based on highest five-year average pensionable earnings. After retirement, pensions are indexed to inflation. Membership in the Pension Plan was closed to Management employees who were not eligible or had not irrevocably elected to join the Pension Plan as of September 30, 2015. These employees are eligible to join the DC Plan. Company and employee contributions to the Pension Plan are based on actuarial valuations performed at least every three years. Annual Pension Plan contributions for 2016 of $108 million (2015 – $177 million) were based on an actuarial valuation effective December 31, 2015 (2015 – based on an actuarial valuation effective December 31, 2013) and the level of pensionable earnings. Estimated annual Pension Plan contributions for 2017 and 2018 are approximately $105 million and $102 million, respectively, based on the actuarial valuation as at December 31, 2015 and projected levels of pensionable earnings. Future minimum contributions beyond 2018 will be based on an actuarial valuation effective no later than December 31, 2018. Contributions are payable one month in arrears. All of the contributions are expected to be in the form of cash. The Hydro One Supplemental Pension Plan (Supplemental Plan) provides members of the Pension Plan with benefits that would have been earned and payable under the Pension Plan but for limitations imposed by the Income Tax Act Hydro One recognizes the overfunded or underfunded status of the Pension Plan, and post-retirement and post-employment benefit plans (Plans) as an asset or liability on its Consolidated Balance Sheets, with offsetting regulatory assets and liabilities as appropriate. The underfunded benefit obligations for the Plans, in the absence of regulatory accounting, would be recognized in AOCI. The impact of changes in assumptions used to measure pension, post-retirement and post-employment benefit obligations is generally recognized over the expected average remaining service period of the employees. The measurement date for the Plans is December 31. Pension Benefits Post-Retirement and Post-Employment Benefits Year ended December 31 (millions of dollars) 2016 2015 2016 2015 Change in projected benefit obligation Projected benefit obligation, beginning of year 7,683 7,535 1,591 1,582 Current service cost 144 146 41 43 Employee contributions 45 40 — — Interest cost 308 302 66 64 Benefits paid (354 ) (334 ) (43 ) (47 ) Net actuarial loss (gain) (52 ) (6 ) 14 (27 ) Change due to Hydro One Brampton spin-off — — — (5 ) Change due to Hydro One Telecom spin-off — — — (19 ) Change due to employees transfer — — 7 — Projected benefit obligation, end of year 7,774 7,683 1,676 1,591 Change in plan assets Fair value of plan assets, beginning of year 6,731 6,299 — — Actual return on plan assets 370 582 — — Benefits paid (354 ) (334 ) (43 ) (47 ) Employer contributions 108 177 43 47 Employee contributions 45 40 — — Administrative expenses (26 ) (33 ) — — Fair value of plan assets, end of year 6,874 6,731 — — Unfunded status 900 952 1,676 1,591 Hydro One presents its benefit obligations and plan assets net on its Consolidated Balance Sheets as follows: Pension Benefits Post-Retirement and Post-Employment Benefits December 31 (millions of dollars) 2016 2015 2016 2015 Other assets 1 1 — — — Accrued liabilities — — 55 50 Pension benefit liability 900 952 — — Post-retirement and post-employment benefit liability — — 1,628 2 1,541 Net unfunded status 899 952 1,683 1,591 1 Represents the funded status of Great Lakes Power’s defined benefit pension plan. 2 Includes $7 million (2015 – $nil) relating to Great Lakes Power’s post-employment benefit plans. The funded or unfunded status of the pension, post-retirement and post-employment benefit plans refers to the difference between the fair value of plan assets and the projected benefit obligations for the Plans. The funded/unfunded status changes over time due to several factors, including contribution levels, assumed discount rates and actual returns on plan assets. The following table provides the projected benefit obligation (PBO), accumulated benefit obligation (ABO) and fair value of plan assets for the Pension Plan: December 31 (millions of dollars) 2016 2015 PBO 7,774 7,683 ABO 7,094 7,020 Fair value of plan assets 6,874 6,731 On an ABO basis, the Pension Plan was funded at 97% at December 31, 2016 (2015 – 96%). On a PBO basis, the Pension Plan was funded at 88% at December 31, 2016 (2015 – 88%). The ABO differs from the PBO in that the ABO includes no assumption about future compensation levels. Components of Net Periodic Benefit Costs The following table provides the components of the net periodic benefit costs for the years ended December 31, 2016 and 2015 for the Pension Plan: Year ended December 31 (millions of dollars) 2016 2015 Current service cost, net of employee contributions 144 146 Interest cost 308 302 Expected return on plan assets, net of expenses (432 ) (406 ) Amortization of actuarial losses 96 119 Prior service cost amortization — 2 Net periodic benefit costs 116 163 Charged to results of operations 1 45 81 1 The Company follows the cash basis of accounting consistent with the inclusion of pension costs in OEB-approved rates. During the year ended December 31, 2016, pension costs of $105 million (2015 – $177 million) were attributed to labour, of which $45 million (2015 – $81 million) was charged to operations, and $60 million (2015 – $96 million) was capitalized as part of the cost of property, plant and equipment and intangible assets. The following table provides the components of the net periodic benefit costs for the years ended December 31, 2016 and 2015 for the post-retirement and post-employment benefit plans: Year ended December 31 (millions of dollars) 2016 2015 Current service cost, net of employee contributions 41 43 Interest cost 66 64 Amortization of actuarial losses 15 14 Prior service cost amortization — — Net periodic benefit costs 122 121 Charged to results of operations 53 55 Assumptions The measurement of the obligations of the Plans and the costs of providing benefits under the Plans involves various factors, including the development of valuation assumptions and accounting policy elections. When developing the required assumptions, the Company considers historical information as well as future expectations. The measurement of benefit obligations and costs is impacted by several assumptions including the discount rate applied to benefit obligations, the long-term expected rate of return on plan assets, Hydro One’s expected level of contributions to the Plans, the incidence of mortality, the expected remaining service period of plan participants, the level of compensation and rate of compensation increases, employee age, length of service, and the anticipated rate of increase of health care costs, among other factors. The impact of changes in assumptions used to measure the obligations of the Plans is generally recognized over the expected average remaining service period of the plan participants. In selecting the expected rate of return on plan assets, Hydro One considers historical economic indicators that impact asset returns, as well as expectations regarding future long-term capital market performance, weighted by target asset class allocations. In general, equity securities, real estate and private equity investments are forecasted to have higher returns than fixed-income securities. The following weighted average assumptions were used to determine the benefit obligations at December 31, 2016 and 2015: Pension Benefits Post-Retirement and Post-Employment Benefits Year ended December 31 2016 2015 2016 2015 Significant assumptions: Weighted average discount rate 3.90 % 4.00 % 3.90 % 4.10 % Rate of compensation scale escalation (long-term) 2.50 % 2.50 % 2.50 % 2.50 % Rate of cost of living increase 2.00 % 2.00 % 2.00 % 2.00 % Rate of increase in health care cost trends 1 — — 4.36 % 4.36 % 1 6.25% per annum in 2017, grading down to 4.36% per annum in and after 2031 (2015 – 6.38% in 2016, grading down to 4.36% per annum in and after 2031). The following weighted average assumptions were used to determine the net periodic benefit costs for the years ended December 31, 2016 and 2015. Assumptions used to determine current year-end benefit obligations are the assumptions used to estimate the subsequent year’s net periodic benefit costs. Year ended December 31 2016 2015 Pension Benefits: Weighted average expected rate of return on plan assets 6.50 % 6.50 % Weighted average discount rate 4.00 % 4.00 % Rate of compensation scale escalation (long-term) 2.50 % 2.50 % Rate of cost of living increase 2.00 % 2.00 % Average remaining service life of employees (years) 15 13 Post-Retirement and Post-Employment Benefits: Weighted average discount rate 4.10 % 4.00 % Rate of compensation scale escalation (long-term) 2.50 % 2.50 % Rate of cost of living increase 2.00 % 2.00 % Average remaining service life of employees (years) 15.3 13.8 Rate of increase in health care cost trends 1 4.36 % 4.36 % 1 6.38% per annum in 2016, grading down to 4.36% per annum in and after 2031 (2015 – 6.52% in 2015, grading down to 4.36% per annum in and after 2031). The discount rate used to determine the current year pension obligation and the subsequent year’s net periodic benefit costs is based on a yield curve approach. Under the yield curve approach, expected future benefit payments for each plan are discounted by a rate on a third-party bond yield curve corresponding to each duration. The yield curve is based on “AA” long-term corporate bonds. A single discount rate is calculated that would yield the same present value as the sum of the discounted cash flows. The effect of a 1% change in health care cost trends on the projected benefit obligation for the post-retirement and post-employment benefits at December 31, 2016 and 2015 is as follows: December 31 (millions of dollars) 2016 2015 Projected benefit obligation: Effect of a 1% increase in health care cost trends 286 252 Effect of a 1% decrease in health care cost trends (219 ) (196 ) The effect of a 1% change in health care cost trends on the service cost and interest cost for the post-retirement and post-employment benefits for the years ended December 31, 2016 and 2015 is as follows: Year ended December 31 (millions of dollars) 2016 2015 Service cost and interest cost: Effect of a 1% increase in health care cost trends 22 22 Effect of a 1% decrease in health care cost trends (16 ) (16 ) The following approximate life expectancies were used in the mortality assumptions to determine the projected benefit obligations for the pension and post-retirement and post-employment plans at December 31, 2016 and 2015: December 31, 2016 December 31, 2015 Life expectancy at 65 for a member currently at Life expectancy at 65 for a member currently at Age 65 Age 45 Age 65 Age 45 Male Female Male Female Male Female Male Female 22 24 23 24 23 25 24 26 Estimated Future Benefit Payments At December 31, 2016, estimated future benefit payments to the participants of the Plans were: (millions of dollars) Pension Benefits Post-Retirement and Post-Employment Benefits 2017 321 55 2018 331 57 2019 340 60 2020 349 61 2021 358 65 2022 through to 2026 1,910 353 Total estimated future benefit payments through to 2026 3,609 651 Components of Regulatory Assets A portion of actuarial gains and losses and prior service costs is recorded within regulatory assets on Hydro One’s Consolidated Balance Sheets to reflect the expected regulatory inclusion of these amounts in future rates, which would otherwise be recorded in OCI. The following table provides the actuarial gains and losses and prior service costs recorded within regulatory assets: Year ended December 31 (millions of dollars) 2016 2015 Pension Benefits: Actuarial loss (gain) for the year 35 (181 ) Amortization of actuarial losses (96 ) (119 ) Prior service cost amortization — (2 ) (61 ) (302 ) Post-Retirement and Post-Employment Benefits: Actuarial loss (gain) for the year 14 (27 ) Amortization of actuarial losses (15 ) (14 ) Prior service cost amortization — — (1 ) (41 ) The following table provides the components of regulatory assets that have not been recognized as components of net periodic benefit costs for the years ended December 31, 2016 and 2015: Year ended December 31 (millions of dollars) 2016 2015 Pension Benefits: Prior service cost — — Actuarial loss 900 952 900 952 Post-Retirement and Post-Employment Benefits: Actuarial loss 243 240 243 240 The following table provides the components of regulatory assets at December 31 that are expected to be amortized as components of net periodic benefit costs in the following year: Pension Benefits Post-Retirement and Post-Employment Benefits December 31 (millions of dollars) 2016 2015 2016 2015 Prior service cost — — — — Actuarial loss 79 96 6 8 79 96 6 8 Pension Plan Assets Investment Strategy On a regular basis, Hydro One evaluates its investment strategy to ensure that Pension Plan assets will be sufficient to pay Pension Plan benefits when due. As part of this ongoing evaluation, Hydro One may make changes to its targeted asset allocation and investment strategy. The Pension Plan is managed at a net asset level. The main objective of the Pension Plan is to sustain a certain level of net assets in order to meet the pension obligations of the Company. The Pension Plan fulfills its primary objective by adhering to specific investment policies outlined in its Summary of Investment Policies and Procedures (SIPP), which is reviewed and approved by the Human Resource Committee of Hydro One’s Board of Directors. The Company manages net assets by engaging knowledgeable external investment managers who are charged with the responsibility of investing existing funds and new funds (current year’s employee and employer contributions) in accordance with the approved SIPP. The performance of the managers is monitored through a governance structure. Increases in net assets are a direct result of investment income generated by investments held by the Pension Plan and contributions to the Pension Plan by eligible employees and by the Company. The main use of net assets is for benefit payments to eligible Pension Plan members. Pension Plan Asset Mix At December 31, 2016, the Pension Plan target asset allocations and weighted average asset allocations were as follows: Target Allocation (%) Pension Plan Assets (%) Equity securities 55.0 58.7 Debt securities 35.0 33.6 Other 1 10.0 7.7 100.0 100.0 1 Other investments include real estate and infrastructure investments. At December 31, 2016, the Pension Plan held $11 million (2015 – $9 million) Hydro One corporate bonds and $450 million (2015 – $420 million) of debt securities of the Province. Concentrations of Credit Risk Hydro One evaluated its Pension Plan’s asset portfolio for the existence of significant concentrations of credit risk as at December 31, 2016 and 2015. Concentrations that were evaluated include, but are not limited to, investment concentrations in a single entity, concentrations in a type of industry, and concentrations in individual funds. At December 31, 2016 and 2015, there were no significant concentrations (defined as greater than 10% of plan assets) of risk in the Pension Plan’s assets. The Pension Plan manages its counterparty credit risk with respect to bonds by investing in investment-grade and government bonds and with respect to derivative instruments by transacting only with financial institutions rated at least “A+” by Standard & Poor’s Rating Services, DBRS Limited, and Fitch Ratings Inc., and “A1” by Moody’s Investors Service, and also by utilizing exposure limits to each counterparty and ensuring that exposure is diversified across counterparties. The risk of default on transactions in listed securities is considered minimal, as the trade will fail if either party to the transaction does not meet its obligation. Fair Value Measurements The following tables present the Pension Plan assets measured and recorded at fair value on a recurring basis and their level within the fair value hierarchy at December 31, 2016 and 2015: December 31, 2016 (millions of dollars) Level 1 Level 2 Level 3 Total Pooled funds — 20 425 445 Cash and cash equivalents 146 — — 146 Short-term securities — 127 — 127 Corporate shares – Canadian 911 — — 911 Corporate shares – Foreign 2,985 113 — 3,098 Bonds and debentures – Canadian — 1,943 — 1,943 Bonds and debentures – Foreign — 193 — 193 Total fair value of plan assets 1 4,042 2,396 425 6,863 1 At December 31, 2016, the total fair value of Pension Plan assets excludes $27 million of interest and dividends receivable, $15 million of purchased investments payable, $9 million of pension administration expenses payable, and $7 million of sold investments receivable. December 31, 2015 (millions of dollars) Level 1 Level 2 Level 3 Total Pooled funds — 23 301 324 Cash and cash equivalents 191 — — 191 Short-term securities — 80 — 80 Corporate shares – Canadian 807 — — 807 Corporate shares – Foreign 2,931 116 — 3,047 Bonds and debentures – Canadian — 2,072 — 2,072 Bonds and debentures – Foreign — 201 — 201 Total fair value of plan assets 1 3,929 2,492 301 6,722 1 At December 31, 2015, the total fair value of Pension Plan assets excludes $27 million of interest and dividends receivable, and $18 million relating to accruals for pension administration expense and foreign exchange contracts payable. See note 16 – Fair Value of Financial Instruments and Risk Management for a description of levels within the fair value hierarchy. Changes in the Fair Value of Financial Instruments Classified in Level 3 The following table summarizes the changes in fair value of financial instruments classified in Level 3 for the years ended December 31, 2016 and 2015. The Pension Plan classifies financial instruments as Level 3 when the fair value is measured based on at least one significant input that is not observable in the markets or due to lack of liquidity in certain markets. The gains and losses presented in the table below may include changes in fair value based on both observable and unobservable inputs. Year ended December 31 (millions of dollars) 2016 2015 Fair value, beginning of year 301 144 Realized and unrealized gains 23 51 Purchases 151 106 Sales and disbursements (50 ) — Fair value, end of year 425 301 There were no significant transfers between any of the fair value levels during the years ended December 31, 2016 and 2015. The Company performs sensitivity analysis for fair value measurements classified in Level 3, substituting the unobservable inputs with one or more reasonably possible alternative assumptions. These sensitivity analyses resulted in negligible changes in the fair value of financial instruments classified in this level. Valuation Techniques Used to Determine Fair Value Pooled funds mainly consist of private equity, real estate and infrastructure investments. Private equity investments represent private equity funds that invest in operating companies that are not publicly traded on a stock exchange. Investment strategies in private equity include limited partnerships in businesses that are characterized by high internal growth and operational efficiencies, venture capital, leveraged buyouts and special situations such as distressed investments. Real estate and infrastructure investments represent funds that invest in real assets which are not publicly traded on a stock exchange. Investment strategies in real estate include limited partnerships that seek to generate a total return through income and capital growth by investing primarily in global and Canadian limited partnerships. Investment strategies in infrastructure include limited partnerships in core infrastructure assets focusing on assets that generate stable, long-term cash flows and deliver incremental returns relative to conventional fixed-income investments. Private equity, real estate and infrastructure valuations are reported by the fund manager and are based on the valuation of the underlying investments which includes inputs such as cost, operating results, discounted future cash flows and market-based comparable data. Since these valuation inputs are not highly observable, private equity and infrastructure investments have been categorized as Level 3 within pooled funds. Cash equivalents consist of demand cash deposits held with banks and cash held by the investment managers. Cash equivalents are categorized as Level 1. Short-term securities are valued at cost plus accrued interest, which approximates fair value due to their short-term nature. Short-term securities are categorized as Level 2. Corporate shares are valued based on quoted prices in active markets and are categorized as Level 1. Investments denominated in foreign currencies are translated into Canadian currency at year-end rates of exchange. Bonds and debentures are presented at published closing trade quotations, and are categorized as Level 2.

Environmental Liabilities

Environmental Liabilities	12 Months Ended
	Dec. 31, 2016
Text Block [Abstract]
Environmental Liabilities	19. ENVIRONMENTAL LIABILITIES The following tables show the movements in environmental liabilities for the years ended December 31, 2016 and 2015: Year ended December 31, 2016 (millions of dollars) PCB Land Assessment Total Environmental liabilities, January 1 148 59 207 Interest accretion 7 1 8 Expenditures (11 ) (9 ) (20 ) Revaluation adjustment (1 ) 10 9 Environmental liabilities, December 31 143 61 204 Less: current portion 18 9 27 125 52 177 Year ended December 31, 2015 (millions of dollars) PCB Land Assessment Total Environmental liabilities, January 1 172 67 239 Interest accretion 8 2 10 Expenditures (8 ) (11 ) (19 ) Revaluation adjustment (24 ) 1 (23 ) Environmental liabilities, December 31 148 59 207 Less: current portion 12 10 22 136 49 185 The following tables show the reconciliation between the undiscounted basis of the environmental liabilities and the amount recognized on the Consolidated Balance Sheets after factoring in the discount rate: December 31, 2016 (millions of dollars) PCB Land Assessment Total Undiscounted environmental liabilities 158 66 224 Less: discounting accumulated liabilities to present value 15 5 20 Discounted environmental liabilities 143 61 204 December 31, 2015 (millions of dollars) PCB Land Assessment Total Undiscounted environmental liabilities 168 61 229 Less: discounting accumulated liabilities to present value 20 2 22 Discounted environmental liabilities 148 59 207 At December 31, 2016, the estimated future environmental expenditures were as follows: (millions of dollars) 2017 27 2018 26 2019 25 2020 29 2021 36 Thereafter 81 224 Hydro One records a liability for the estimated future expenditures for land assessment and remediation and for the phase-out and destruction of PCB-contaminated mineral oil removed from electrical equipment when it is determined that future environmental remediation expenditures are probable under existing statute or regulation and the amount of the future expenditures can be reasonably estimated. There are uncertainties in estimating future environmental costs due to potential external events such as changes in legislation or regulations, and advances in remediation technologies. In determining the amounts to be recorded as environmental liabilities, the Company estimates the current cost of completing required work and makes assumptions as to when the future expenditures will actually be incurred, in order to generate future cash flow information. A long-term inflation rate assumption of approximately 2% has been used to express these current cost estimates as estimated future expenditures. Future expenditures have been discounted using factors ranging from approximately 2.0% to 6.3%, depending on the appropriate rate for the period when expenditures are expected to be incurred. All factors used in estimating the Company’s environmental liabilities represent management’s best estimates of the present value of costs required to meet existing legislation or regulations. However, it is reasonably possible that numbers or volumes of contaminated assets, cost estimates to perform work, inflation assumptions and the assumed pattern of annual cash flows may differ significantly from the Company’s current assumptions. In addition, with respect to the PCB environmental liability, the availability of critical resources such as skilled labour and replacement assets and the ability to take maintenance outages in critical facilities may influence the timing of expenditures. PCBs The Environment Canada regulations, enacted under the Canadian Environmental Protection Act, 1999 The Company’s best estimate of the total estimated future expenditures to comply with current PCB regulations is $158 million (2015 – $168 million). These expenditures are expected to be incurred over the period from 2017 to 2025. As a result of its annual review of environmental liabilities, the Company recorded a revaluation adjustment in 2016 to reduce the PCB environmental liability by $1 million (2015 – $24 million). Land Assessment and Remediation The Company’s best estimate of the total estimated future expenditures to complete its land assessment and remediation program is $66 million (2015 – $61 million). These expenditures are expected to be incurred over the period from 2017 to 2032. As a result of its annual review of environmental liabilities, the Company recorded a revaluation adjustment in 2016 to increase the land assessment and remediation environmental liability by $10 million (2015 – $1 million).

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Dec. 31, 2016
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligations	20. ASSET RETIREMENT OBLIGATIONS Hydro One records a liability for the estimated future expenditures for the removal and disposal of asbestos-containing materials installed in some of its facilities and for the decommissioning of specific switching stations located on unowned sites. Asset retirement obligations, which represent legal obligations associated with the retirement of certain tangible long-lived assets, are computed as the present value of the projected expenditures for the future retirement of specific assets and are recognized in the period in which the liability is incurred, if a reasonable estimate of fair value can be made. If the asset remains in service at the recognition date, the present value of the liability is added to the carrying amount of the associated asset in the period the liability is incurred and this additional carrying amount is depreciated over the remaining life of the asset. If an asset retirement obligation is recorded in respect of an out-of-service asset, the asset retirement cost is charged to results of operations. Subsequent to the initial recognition, the liability is adjusted for any revisions to the estimated future cash flows associated with the asset retirement obligation, which can occur due to a number of factors including, but not limited to, cost escalation, changes in technology applicable to the assets to be retired, changes in legislation or regulations, as well as for accretion of the liability due to the passage of time until the obligation is settled. Depreciation expense is adjusted prospectively for any increases or decreases to the carrying amount of the associated asset. In determining the amounts to be recorded as asset retirement obligations, the Company estimates the current fair value for completing required work and makes assumptions as to when the future expenditures will actually be incurred, in order to generate future cash flow information. A long-term inflation assumption of approximately 2% has been used to express these current cost estimates as estimated future expenditures. Future expenditures have been discounted using factors ranging from approximately 3.0% to 5.0%, depending on the appropriate rate for the period when expenditures are expected to be incurred. All factors used in estimating the Company’s asset retirement obligations represent management’s best estimates of the cost required to meet existing legislation or regulations. However, it is reasonably possible that numbers or volumes of contaminated assets, cost estimates to perform work, inflation assumptions and the assumed pattern of annual cash flows may differ significantly from the Company’s current assumptions. Asset retirement obligations are reviewed annually or more frequently if significant changes in regulations or other relevant factors occur. Estimate changes are accounted for prospectively. At December 31, 2016, Hydro One had recorded asset retirement obligations of $9 million (2015 – $9 million), primarily consisting of the estimated future expenditures associated with the removal and disposal of asbestos-containing materials installed in some of its facilities. The amount of interest recorded is nominal.

Share Capital

Share Capital	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Share Capital	21. SHARE CAPITAL Common Shares The Company is authorized to issue an unlimited number of common shares. At December 31, 2016, the Company had 142,239 common shares issued and outstanding (2015 – 142,239). In 2016, a return of stated capital in the amount of $609 million (2015 – $nil) was paid. The amount and timing of any dividends payable by Hydro One is at the discretion of the Hydro One Board of Directors and is established on the basis of Hydro One’s results of operations, maintenance of its deemed regulatory capital structure, financial condition, cash requirements, the satisfaction of solvency tests imposed by corporate laws for the declaration and payment of dividends and other factors that the Board of Directors may consider relevant. Preferred Shares The Company is authorized to issue an unlimited number of preferred shares, issuable in series. At December 31, 2016, Hydro One had no issued and outstanding preferred shares. On November 2, 2015, a special resolution of Hydro One Limited (as sole shareholder of Hydro One) was made to amend the articles of Hydro One to delete the share ownership restrictions and to amend the Hydro One preferred share terms to provide for basic redeemable preferred shares. When issued, the Class A preferred shares will be redeemable at the option of the Company. The holders of the Class A preferred shares will be entitled to receive, if and when declared by the Hydro One Board of Directors, non-cumulative preferred share dividends at a rate per year to be determined by the Hydro One Board of Directors. The holders of the Class A preferred shares will not be entitled to receive notice of, or to attend or to vote at, any meeting of the shareholders of Hydro One. The holders of the Class A preferred shares will be entitled to receive, before any distributions to the holders of common shares and any other shares ranking junior to the Class A preferred shares, an amount equal to the amount paid for the Class A preferred shares together with all dividends declared and unpaid up to the date of liquidation, dissolution or winding up of Hydro One, or the date of redemption. Prior to October 31, 2015, the Company had 12,920,000 issued and outstanding 5.5% cumulative preferred shares held by the Province, with a redemption value of $25 per share or $323 million total value. These preferred shares were entitled to an annual cumulative dividend of $18 million, or $1.375 per share, which was payable on a quarterly basis. These preferred shares had conditions for their redemption that were outside the control of the Company because the Province could exercise its right to redeem in the event of change in ownership without approval of the Company’s Board of Directors. At December 31, 2014, these preferred shares were classified on the Consolidated Balance Sheet as temporary equity because the redemption feature was outside the control of the Company. On October 31, 2015, these preferred shares were purchased and cancelled by Hydro One. See “Reorganization” below for further details. Reorganization Prior to the completion of the IPO, Hydro One and Hydro One Limited completed a series of transactions (Pre-IPO Transactions) that resulted in, among other things, on October 31, 2015, Hydro One Limited acquiring all of the issued and outstanding shares of Hydro One from the Province. The following table presents the common shares issued during the year ended December 31, 2015. Year ended December 31, 2015 (millions of dollars) (number of shares) Pre-Closing Transactions: Common shares issued – purchase and cancellation of preferred shares (a) 323 2,640 Common shares issued (b) 2,600 39,598 Common shares issued (c) — 1 Total common shares issued 2,923 42,239 (a) On October 31, 2015, Hydro One purchased and cancelled its 12,920,000 preferred shares previously held by the Province for cancellation at a price equal to the redemption price of the preferred shares totalling $323 million, which was satisfied by the issuance to the Province of 2,640 common shares of Hydro One. (b) On November 4, 2015, Hydro One issued 39,598 common shares to Hydro One Limited for proceeds of $2.6 billion. (c) On November 3, 2015, Hydro One declared a stock dividend on its common shares, which due to the number of shares issued and the resulting effect on the price per share was treated as a stock split. On November 5, 2015, Hydro One effected a reverse split and issued as consideration one common share to Hydro One Limited. There was no impact to the capital structure of Hydro One as a net result of the stock dividend and the reverse split.

Dividends

Dividends	12 Months Ended
	Dec. 31, 2016
Text Block [Abstract]
Dividends	22. DIVIDENDS In 2016, preferred share dividends in the amount of $nil (2015 – $13 million) and common share dividends in the amount of $2 million (2015 – $875 million) were declared. In August 2015, Hydro One declared a dividend in-kind on its common shares payable in all of the issued and outstanding shares of Hydro One Brampton (see note 4).

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2016
Earnings Per Share [Abstract]
Earnings Per Share	23. EARNINGS PER SHARE Basic and diluted earnings per common share (EPS) is calculated by dividing net income attributable to common shareholder of Hydro One by the weighted average number of common shares outstanding. The weighted average number of shares outstanding at December 31, 2016 was 142,239 (2015 – 107,116). There were no dilutive securities during 2016 or 2015.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	24. STOCK-BASED COMPENSATION The following compensation plans were established by Hydro One Limited, however they represent components of compensation costs of Hydro One in current and future periods. Share Grant Plans At December 31, 2016, Hydro One Limited had two share grant plans (Share Grant Plans), one for the benefit of certain members of the Power Workers’ Union (the PWU Share Grant Plan) and one for the benefit of certain members of The Society of Energy Professionals (the Society Share Grant Plan). Hydro One and Hydro One Limited entered into an intercompany agreement, such that Hydro One will pay Hydro One Limited for the compensation costs associated with these plans. The PWU Share Grant Plan provides for the issuance of common shares of Hydro One Limited from treasury to certain eligible members of the Power Workers’ Union annually, commencing on April 1, 2017 and continuing until the earlier of April 1, 2028 or the date an eligible employee no longer meets the eligibility criteria of the PWU Share Grant Plan. To be eligible, an employee must be a member of the Pension Plan on April 1, 2015, be employed on the date annual share issuance occurs and continue to have under 35 years of service. The requisite service period for the PWU Share Grant Plan begins on July 3, 2015, which is the date the share grant plan was ratified by the PWU. The number of common shares issued annually to each eligible employee will be equal to 2.7% of such eligible employee’s salary as at April 1, 2015, divided by $20.50, being the price of the common shares of Hydro One Limited in the IPO. The aggregate number of Hydro One Limited common shares issuable under the PWU Share Grant Plan shall not exceed 3,981,763 common shares. In 2015, 3,952,212 Hydro One Limited common shares were granted under the PWU Share Grant Plan relevant to the total share based compensation recognized by Hydro One. The Society Share Grant Plan provides for the issuance of common shares of Hydro One Limited from treasury to certain eligible members of The Society of Energy Professionals annually, commencing on April 1, 2018 and continuing until the earlier of April 1, 2029 or the date an eligible employee no longer meets the eligibility criteria of the Society Share Grant Plan. To be eligible, an employee must be a member of the Pension Plan on September 1, 2015, be employed on the date annual share issuance occurs and continue to have under 35 years of service. Therefore the requisite service period for the Society Share Grant Plan begins on September 1, 2015. The number of common shares issued annually to each eligible employee will be equal to 2.0% of such eligible employee’s salary as at September 1, 2015, divided by $20.50, being the price of the common shares of Hydro One Limited in the IPO. The aggregate number of Hydro One Limited common shares issuable under the Society Share Grant Plan shall not exceed 1,434,686 common shares. In 2015, 1,367,158 Hydro One Limited common shares were granted under the Society Share Grant Plan relevant to the total share based compensation recognized by Hydro One. The fair value of the Hydro One Limited 2015 share grants of $111 million was estimated based on the grant date Hydro One Limited share price of $20.50 and is recognized using the graded-vesting attribution method as the share grant plans have both a performance condition and a service condition. No shares were granted under the Share Grant Plans in 2016. Total share based compensation recognized during 2016 was $21 million (2015 – $10 million) and was recorded as a regulatory asset. A summary of share grant activity under the Share Grant Plans during years ended December 31, 2016 and 2015 is presented below: Year ended December 31, 2016 Share Grants (number of common shares) Weighted-Average Price Share grants outstanding – January 1, 2016 5,319,370 $ 20.50 Granted (non-vested) — — Forfeited 1 (79,692 ) $ 20.50 Share grants outstanding – December 31, 2016 5,239,678 $ 20.50 1 Includes shares forfeited as well as shares transferred corresponding to transfer of employees from an affiliate company. Year ended December 31, 2015 Share Grants (number of common shares) Weighted-Average Price Share grants outstanding – January 1, 2015 — — Granted (non-vested) 5,319,370 $ 20.50 Share grants outstanding – December 31, 2015 5,319,370 $ 20.50 Directors’ DSU Plan Under the Directors’ DSU Plan, directors can elect to receive credit for their annual cash retainer in a notional account of DSUs in lieu of cash. Hydro One Limited’s Board of Directors may also determine from time to time that special circumstances exist that would reasonably justify the grant of DSUs to a director as compensation in addition to any regular retainer or fee to which the director is entitled. Each DSU represents a unit with an underlying value equivalent to the value of one common share of Hydro One Limited and is entitled to accrue Hydro One Limited common share dividend equivalents in the form of additional DSUs at the time dividends are paid, subsequent to declaration by Hydro One Limited’s Board of Directors. Year ended December 31 (number of DSUs) 2016 2015 DSUs outstanding – January 1 20,525 — DSUs granted 78,558 20,525 DSUs outstanding – December 31 99,083 20,525 For the year ended December 31, 2016, an expense of $2 million (2015 – less than $1 million) was recognized in earnings with respect to the DSU Plan. At December 31, 2016, a liability of $2 million (December 31, 2015 – less than $1 million), related to outstanding DSUs has been recorded at the closing price of Hydro One Limited’s common shares of $23.58 and is included in accrued liabilities on the Consolidated Balance Sheets. Employee Share Ownership Plan Effective December 15, 2015, Hydro One Limited established an Employee Share Ownership Plan (ESOP). Under the ESOP, certain eligible management and non-represented employees may contribute between 1% and 6% of their base salary towards purchasing common shares of Hydro One Limited. The Company matches 50% of the employee’s contributions, up to a maximum Company contribution of $25,000 per calendar year. In 2016, Company contributions made under the ESOP were $2 million (2015—$nil). Long-term Incentive Plan Effective August 31, 2015, the Board of Directors of Hydro One Limited adopted an LTIP. Under the LTIP, long-term incentives are granted to certain executive and management employees of Hydro One Limited and its subsidiaries, and all equity-based awards will be settled in newly issued shares of Hydro One Limited from treasury, consistent with the provisions of the plan. The aggregate number of shares issuable under the LTIP shall not exceed 11,900,000 shares of Hydro One Limited. The LTIP provides flexibility to award a range of vehicles, including restricted share units (RSUs), performance share units (PSUs), stock options, share appreciation rights, restricted shares, deferred share units and other share-based awards. The mix of vehicles is intended to vary by role to recognize the level of executive accountability for overall business performance. During 2016, Hydro One Limited granted awards under its LTIP, consisting of PSUs and RSUs, all of which are equity settled in Hydro One Limited shares, as follows: Year ended December 31, 2016 Number of PSUs Number of RSUs Units outstanding – January 1, 2016 — — Units granted 233,710 257,260 Units forfeited (4,820 ) (4,820 ) Units outstanding – December 31, 2016 228,890 252,440 The grant date total fair value of the awards was $12 million (2015 – $nil). The compensation expense recognized by the Company relating to these awards during 2016 was $3 million (2015 – $nil).

Noncontrolling Interest

Noncontrolling Interest	12 Months Ended
	Dec. 31, 2016
Noncontrolling Interest [Abstract]
Noncontrolling Interest	25. NONCONTROLLING INTEREST On December 16, 2014, transmission assets totalling $526 million were transferred from Hydro One Networks to B2M LP. This was financed by 60% debt ($316 million) and 40% equity ($210 million). On December 17, 2014, the Saugeen Ojibway Nation (SON) acquired a 34.2% equity interest in B2M LP for consideration of $72 million, representing the fair value of the equity interest acquired. The SON’s initial investment in B2M LP consists of $50 million of Class A units and $22 million of Class B units. The Class B units have a mandatory put option which requires that upon the occurrence of an enforcement event (i.e. an event of default such as a debt default by the SON or insolvency event), Hydro One purchase the Class B units of B2M LP for net book value on the redemption date. The noncontrolling interest relating to the Class B units is classified on the Consolidated Balance Sheet as temporary equity because the redemption feature is outside the control of the Company. The balance of the noncontrolling interest is classified within equity. The following tables show the movements in noncontrolling interest for the years ended December 31, 2016 and 2015: Year ended December 31, 2016 (millions of dollars) Temporary Equity Equity Total Noncontrolling interest – January 1, 2016 23 52 75 Distributions to noncontrolling interest (3 ) (6 ) (9 ) Net income attributable to noncontrolling interest 2 4 6 Noncontrolling interest – December 31, 2016 22 50 72 Year ended December 31, 2015 (millions of dollars) Temporary Equity Equity Total Noncontrolling interest – January 1, 2015 21 49 70 Distributions to noncontrolling interest (1 ) (4 ) (5 ) Net income attributable to noncontrolling interest 3 7 10 Noncontrolling interest – December 31, 2015 23 52 75

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2016
Related Party Transactions [Abstract]
Related Party Transactions	26. RELATED PARTY TRANSACTIONS Hydro One is owned by Hydro One Limited. The Province is the majority shareholder of Hydro One Limited. The IESO, Ontario Power Generation Inc. (OPG), OEFC, OEB, Hydro One Brampton and Hydro One Telecom are related parties to Hydro One because they are controlled or significantly influenced by the Province or by Hydro One Limited. Year ended December 31 2016 2015 Related Party Transaction (millions of dollars) Province 1 Dividends paid — 888 IESO Power purchased 2,096 2,318 Revenues for transmission services 1,549 1,548 Distribution revenues related to rural rate protection 125 127 Distribution revenues related to the supply of electricity to remote northern communities 32 32 Funding received related to Conservation and Demand Management programs 63 70 OPG Power purchased 6 11 Revenues related to provision of construction and equipment maintenance services 4 7 Costs expensed related to the purchase of services 1 1 OEFC Payments in lieu of corporate income taxes 2 — 2,933 Power purchased from power contracts administered by the OEFC 1 6 Indemnification fee paid (terminated effective October 31, 2015) — 8 OEB OEB fees 11 12 Hydro One Brampton 1 Revenues from management, administrative and smart meter network services 3 1 Hydro One Limited Common shares issued 3 — 2,600 Return of stated capital 609 — Dividends paid 2 — Stock-based compensation costs 24 10 IPO costs subsequently reimbursed by Hydro One Limited 4 — 7 Hydro One Telecom Services received – costs expensed 24 4 Services received – costs capitalized 12 2 Revenues for services provided 3 — 1 On August 31, 2015, Hydro One completed the spin-off of its subsidiary, Hydro One Brampton, to the Province. 2 In 2015, Hydro One made PILs to the OEFC totalling $2.9 billion, including Departure Tax of $2.6 billion. 3 On November 4, 2015, Hydro One issued 39,598 common shares to Hydro One Limited for proceeds of $2.6 billion. 4 In 2015, Hydro One incurred certain IPO related expenses totalling $7 million, which were subsequently reimbursed to the Company by Hydro One Limited. Sales to and purchases from related parties are based on the requirements of the OEB’s Affiliate Relationships Code. Outstanding balances at period end are interest free and settled in cash. The amounts due to and from related parties as a result of the transactions referred to above are as follows: December 31 (millions of dollars) 2016 2015 Due from related parties 224 184 Due to related parties 1 (279 ) (142 ) 1 Included in due to related parties at December 31, 2016 are amounts owing to the IESO in respect of power purchases of $143 million (2015 – $134 million).

Consolidated Statements of Ca34

Consolidated Statements of Cash Flows	12 Months Ended
	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Consolidated Statements of Cash Flows	27. CONSOLIDATED STATEMENTS OF CASH FLOWS The changes in non-cash balances related to operations consist of the following: Year ended December 31 (millions of dollars) 2016 2015 Accounts receivable (59 ) 249 Due from related parties (40 ) 40 Materials and supplies 2 2 Prepaid expenses and other assets (17 ) 12 Accounts payable 18 (26 ) Accrued liabilities 52 (27 ) Due to related parties 113 (95 ) Accrued interest 9 (4 ) Long-term accounts payable and other liabilities 6 — Post-retirement and post-employment benefit liability 84 41 168 192 Capital Expenditures The following table reconciles between investments in property, plant and equipment and the amount presented in the Consolidated Statements of Cash Flows after accounting for capitalized depreciation and the net change in related accruals: Year ended December 31 (millions of dollars) 2016 2015 Capital investments in property, plant and equipment (1,624 ) (1,622 ) Capitalized depreciation and net change in accruals included in capital investments in property, plant and equipment 30 28 Capital expenditures – property, plant and equipment (1,594 ) (1,594 ) The following table reconciles between investments in intangible assets and the amount presented in the Consolidated Statements of Cash Flows after accounting for the net change in related accruals: Year ended December 31 (millions of dollars) 2016 2015 Capital investments in intangible assets (67 ) (40 ) Net change in accruals included in capital investments in intangible assets 6 3 Capital expenditures – intangible assets (61 ) (37 ) Capital Contributions Hydro One enters into contracts governed by the OEB Transmission System Code when a transmission customer requests a new or upgraded transmission connection. The customer is required to make a capital contribution to Hydro One based on the shortfall between the present value of the costs of the connection facility and the present value of revenues. The present value of revenues is based on an estimate of load forecast for the period of the contract with Hydro One. Once the connection facility is commissioned, in accordance with the OEB Transmission System Code, Hydro One will periodically reassess the estimated of load forecast which will lead to a decrease, or an increase in the capital contributions from the customer. The increase or decrease in capital contributions is recorded directly to fixed assets in service. In 2016, capital contributions from these reassessments totalled $21 million (2015 – $57 million), which represents the difference between the revised load forecast of electricity transmitted compared to the load forecast in the original contract, subject to certain adjustments. Supplementary Information Year ended December 31 (millions of dollars) 2016 2015 Net interest paid 418 416 Income taxes / PILs paid 30 2,928

Contingencies

Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Contingencies	28. CONTINGENCIES Legal Proceedings Hydro One is involved in various lawsuits, claims and regulatory proceedings in the normal course of business. In the opinion of management, the outcome of such matters will not have a material adverse effect on the Company’s consolidated financial position, results of operations or cash flows. Hydro One Inc., Hydro One Networks, Hydro One Remote Communities, and Norfolk Power Distribution Inc. are defendants in a class action suit in which the representative plaintiff is seeking up to $125 million in damages related to allegations of improper billing practices. A certification motion in the class action is pending. Due to the preliminary stage of legal proceedings, an estimate of a possible loss related to this claim cannot be made. Transfer of Assets The transfer orders by which the Company acquired certain of Ontario Hydro’s businesses as of April 1, 1999 did not transfer title to some assets located on Reserves (as defined in the Indian Act one-time

Commitments

Commitments	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments	29. COMMITMENTS The following table presents a summary of Hydro One’s commitments under leases, outsourcing and other agreements due in the next 5 years and thereafter. December 31, 2016 (millions of dollars) 2017 2018 2019 2020 2021 Thereafter Outsourcing agreements 165 102 94 2 2 9 Long-term software/meter agreement 17 17 16 17 1 5 Operating lease commitments 9 9 4 8 2 2 Outsourcing Agreements Inergi LP (Inergi), an affiliate of Capgemini Canada Inc., provides services to Hydro One, including settlements, source to pay services, pay operations services, information technology, finance and accounting services. The agreement with Inergi for these services expires in December 2019. In addition, Inergi provides customer service operations outsourcing services to Hydro One. The agreement for these services expires in February 2018. Brookfield Global Integrated Solutions (formerly Brookfield Johnson Controls Canada LP) (Brookfield) provides services to Hydro One, including facilities management and execution of certain capital projects as deemed required by the Company. The agreement with Brookfield for these services expires in December 2024. Long-term software/meter agreement Trilliant Holdings Inc. and Trilliant Networks (Canada) Inc. (collectively Trilliant) provide services to Hydro One for the supply, maintenance and support services for smart meters and related hardware and software, including additional software licences, as well as certain professional services. The agreement with Trilliant for these services expires in December 2025, but Hydro One has the option to renew for an additional term of five years at its sole discretion. Operating Leases Hydro One is committed as lessee to irrevocable operating lease contracts for buildings used in administrative and service-related functions. These leases have typical terms of between three and five years, but several leases have lesser or greater terms to address special circumstances and/or opportunities. Renewal options, which are generally prevalent in most leases, have similar terms of three to five years. All leases include a clause to enable upward revision of the rental charge on an annual basis or on renewal according to prevailing market conditions or pre-established rents. There are no restrictions placed upon Hydro One by entering into these leases. During the year ended December 31, 2016, the Company made lease payments totalling $10 million (2015 – $6 million). Other Commitments Prudential Support Purchasers of electricity in Ontario, through the IESO, are required to provide security to mitigate the risk of their default based on their expected activity in the market. As at December 31, 2016, Hydro One provided prudential support to the IESO on behalf of its subsidiaries using parental guarantees of $329 million (2015 – $329 million), and on behalf of a distributor using guarantees of $1 million (2015 – $1 million). In addition, as at December 31, 2016, Hydro One provided letters of credit in the amount of $24 million (2015 – $15 million), including $17 million (2015 – $15 million) to the IESO. The IESO could draw on these guarantees and/or letters of credit if these subsidiaries or distributor fail to make a payment required by a default notice issued by the IESO. The maximum potential payment is the face value of any letters of credit plus the amount of the parental guarantees. Retirement Compensation Arrangements Bank letters of credit have been issued to provide security for the Company’s liability under the terms of a trust fund established pursuant to the supplementary pension plan for eligible employees of Hydro One. The supplementary pension plan trustee is required to draw upon these letters of credit if Hydro One is in default of its obligations under the terms of this plan. Such obligations include the requirement to provide the trustee with an annual actuarial report as well as letters of credit sufficient to secure the Company’s liability under the plan, to pay benefits payable under the plan and to pay the letter of credit fee. The maximum potential payment is the face value of the letters of credit. At December 31, 2016, Hydro One had letters of credit of $150 million (2015 – $139 million) outstanding relating to retirement compensation arrangements.

Segmented Reporting

Segmented Reporting	12 Months Ended
	Dec. 31, 2016
Segment Reporting [Abstract]
Segmented Reporting	30. SEGMENTED REPORTING Hydro One has three reportable segments: • The Transmission Business, which comprises the transmission of high voltage electricity across the province, interconnecting more than 70 local distribution companies and certain large directly connected industrial customers throughout the Ontario electricity grid; • The Distribution Business, which comprises the delivery of electricity to end customers and certain other municipal electricity distributors; and • Other Business, which includes certain corporate activities. The comparative information also includes the operations of Hydro One Telecom up to November 4, 2015. The designation of segments has been based on a combination of regulatory status and the nature of the services provided. Operating segments of the Company are determined based on information used by the chief operating decision maker in deciding how to allocate resources and evaluate the performance of each of the segments. The Company evaluates segment performance based on income before financing charges and income taxes from continuing operations (excluding certain allocated corporate governance costs). The accounting policies followed by the segments are the same as those described in the summary of significant accounting policies (see note 2). Year ended December 31, 2016 (millions of dollars) Transmission Distribution Other Consolidated Revenues 1,587 4,915 — 6,502 Purchased power — 3,427 — 3,427 Operation, maintenance and administration 410 613 20 1,043 Depreciation and amortization 390 379 — 769 Income (loss) before financing charges and income taxes 787 496 (20 ) 1,263 Capital investments 988 703 — 1,691 Year ended December 31, 2015 (millions of dollars) Transmission Distribution Other Consolidated Revenues 1,536 4,949 44 6,529 Purchased power — 3,450 — 3,450 Operation, maintenance and administration 415 633 82 1,130 Depreciation and amortization 374 380 3 757 Income (loss) before financing charges and income taxes 747 486 (41 ) 1,192 Capital investments 943 711 8 1,662 Total Assets by Segment: December 31 (millions of dollars) 2016 2015 Transmission 13,083 12,045 Distribution 9,393 9,200 Other 2,834 2,924 Total assets 25,310 24,169 All revenues, costs and assets, as the case may be, are earned, incurred or held in Canada.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2016
Subsequent Events [Abstract]
Subsequent Events	31. SUBSEQUENT EVENTS Return of Stated Capital On February 9, 2017, a return of stated capital in the amount of $147 million was approved.

Significant Accounting Polici39

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Basis of Consolidation	Basis of Consolidation These Consolidated Financial Statements include the accounts of the Company and its subsidiaries. Intercompany transactions and balances have been eliminated.
Basis of Accounting	Basis of Accounting These Consolidated Financial Statements are prepared and presented in accordance with United States (US) Generally Accepted Accounting Principles (GAAP) and in Canadian dollars.
Use of Management Estimates	Use of Management Estimates The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities at the date of the financial statements and the reported amounts of revenues, expenses, gains and losses during the reporting periods. Management evaluates these estimates on an ongoing basis based upon historical experience, current conditions, and assumptions believed to be reasonable at the time the assumptions are made, with any adjustments being recognized in results of operations in the period they arise. Significant estimates relate to regulatory assets and regulatory liabilities, environmental liabilities, pension benefits, post-retirement and post-employment benefits, asset retirement obligations, goodwill and asset impairments, contingencies, unbilled revenues, allowance for doubtful accounts, derivative instruments, and deferred income tax assets and liabilities. Actual results may differ significantly from these estimates.
Rate Setting	Rate Setting The Company’s Transmission Business includes the transmission business of Hydro One Networks Inc. (Hydro One Networks), Hydro One Sault Ste. Marie LP (previously Great Lakes Power Transmission LP (Great Lakes Power)), and its 66% interest in B2M Limited Partnership (B2M LP). The Company’s Distribution Business includes the distribution businesses of Hydro One Networks, as well as Hydro One Remote Communities Inc. (Hydro One Remote Communities). Transmission In November 2015, the OEB approved Hydro One Networks’ 2016 transmission rates revenue requirement of $1,480 million. In December 2015, the OEB approved B2M LP’s 2015-2019 rates revenue requirements of $39 million, $36 million, $37 million, $38 million and $37 million for the respective years. On January 14, 2016, the OEB approved the B2M LP revenue requirement recovery through the 2016 Uniform Transmission Rates, and the establishment of a deferral account to capture costs of Tax Rate and Rule changes. Distribution In March 2015, the OEB approved Hydro One Networks’ distribution revenue requirements of $1,326 million for 2015, $1,430 million for 2016 and $1,486 million for 2017. The OEB has subsequently approved updated revenue requirements of $1,410 million for 2016 and $1,415 million for 2017. On March 17, 2016, the OEB approved an increase of 2.10% to Hydro One Remote Communities’ basic rates for the distribution and generation of electricity, with an effective date of May 1, 2016.
Regulatory Accounting	Regulatory Accounting The OEB has the general power to include or exclude revenues, costs, gains or losses in the rates of a specific period, resulting in a change in the timing of accounting recognition from that which would have been applied in an unregulated company. Such change in timing involves the application of rate-regulated accounting, giving rise to the recognition of regulatory assets and liabilities. The Company’s regulatory assets represent certain amounts receivable from future customers and costs that have been deferred for accounting purposes because it is probable that they will be recovered in future rates. In addition, the Company has recorded regulatory liabilities that generally represent amounts that are refundable to future customers. The Company continually assesses the likelihood of recovery of each of its regulatory assets and continues to believe that it is probable that the OEB will include its regulatory assets and liabilities in setting of future rates. If, at some future date, the Company judges that it is no longer probable that the OEB will include a regulatory asset or liability in setting future rates, the appropriate carrying amount would be reflected in results of operations in the period that the assessment is made.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include cash and short-term investments with an original maturity of three months or less.
Revenue Recognition	Revenue Recognition Transmission revenues are collected through OEB-approved rates, which are based on an approved revenue requirement that includes a rate of return. Such revenue is recognized as electricity is transmitted and delivered to customers. Distribution revenues attributable to the delivery of electricity are based on OEB-approved distribution rates and are recognized on an accrual basis and include billed and unbilled revenues. Billed revenues are based on electricity delivered as measured from customer meters. At the end of each month, electricity delivered to customers since the date of the last billed meter reading is estimated, and the corresponding unbilled revenue is recorded. The unbilled revenue estimate is affected by energy consumption, weather, and changes in the composition of customer classes. Distribution revenue also includes an amount relating to rate protection for rural, residential, and remote customers, which is received from the Independent Electricity System Operator (IESO) based on a standardized customer rate that is approved by the OEB. Revenues also include amounts related to sales of other services and equipment. Such revenue is recognized as services are rendered or as equipment is delivered. Revenues are recorded net of indirect taxes.
Accounts Receivable and Allowance for Doubtful Accounts	Accounts Receivable and Allowance for Doubtful Accounts Billed accounts receivable are recorded at the invoiced amount, net of allowance for doubtful accounts. Unbilled accounts receivable are recorded at their estimated value. Overdue amounts related to regulated billings bear interest at OEB-approved rates. The allowance for doubtful accounts reflects the Company’s best estimate of losses on billed accounts receivable balances. The Company estimates the allowance for doubtful accounts on billed accounts receivable by applying internally developed loss rates to the outstanding receivable balances by aging category. Loss rates applied to the billed accounts receivable balances are based on historical overdue balances, customer payments and write-offs. Accounts receivable are written-off against the allowance when they are deemed uncollectible. The allowance for doubtful accounts is affected by changes in volume, prices and economic conditions.
Noncontrolling interest	Noncontrolling interest Noncontrolling interest represents the portion of equity ownership in subsidiaries that is not attributable to shareholders of Hydro One. Noncontrolling interest is initially recorded at fair value and subsequently the amount is adjusted for the proportionate share of net income and other comprehensive income attributable to the noncontrolling interest and any dividends or distributions paid to the noncontrolling interest. If a transaction results in the acquisition of all, or part, of a noncontrolling interest in a subsidiary, the acquisition of the noncontrolling interest is accounted for as an equity transaction. No gain or loss is recognized in consolidated net income or comprehensive income as a result of changes in the noncontrolling interest, unless a change results in the loss of control by the Company.
Income Taxes	Income Taxes Prior to the IPO of Hydro One Limited, Hydro One was exempt from tax under the Income Tax Act Taxation Act, 2007 Electricity Act Current and deferred income taxes are computed based on the tax rates and tax laws enacted as at the balance sheet date. Tax benefits associated with income tax positions taken, or expected to be taken, in a tax return are recorded only when the “more-likely-than-not” recognition threshold is satisfied and are measured at the largest amount of benefit that has a greater than 50% likelihood of being realized upon settlement. Management evaluates each position based solely on the technical merits and facts and circumstances of the position, assuming the position will be examined by a taxing authority having full knowledge of all relevant information. Significant management judgment is required to determine recognition thresholds and the related amount of tax benefits to be recognized in the Consolidated Financial Statements. Management re-evaluates tax positions each period using new information about recognition or measurement as it becomes available. Deferred Income Taxes Deferred income taxes are provided for using the liability method. Deferred income taxes are recognized based on the estimated future tax consequences attributable to temporary differences between the carrying amount of assets and liabilities in the Consolidated Financial Statements and their corresponding tax bases. Deferred income tax liabilities are recognized on all taxable temporary differences. Deferred tax assets are recognized to the extent that it is more-likely-than-not that these assets will be realized from taxable income available against which deductible temporary differences can be utilized. Deferred income taxes are calculated at the tax rates that are expected to apply in the period when the liability is settled or the asset is realized, based on the tax rates and tax laws that have been enacted as at the balance sheet date. Deferred income taxes that are not included in the rate-setting process are charged or credited to the Consolidated Statements of Operations and Comprehensive Income. If management determines that it is more-likely-than-not that some or all of a deferred income tax asset will not be realized, a valuation allowance is recorded against the deferred income tax asset to report the net balance at the amount expected to be realized. Previously unrecognized deferred income tax assets are reassessed at each balance sheet date and are recognized to the extent that it has become more-likely-than-not that the tax benefit will be realized. The Company records regulatory assets and liabilities associated with deferred income taxes that will be included in the rate-setting process. The Company uses the flow-through method to account for investment tax credits (ITCs) earned on eligible scientific research and experimental development expenditures, and apprenticeship job creation. Under this method, only non-refundable ITCs are recognized as a reduction to income tax expense.
Materials and Supplies	Materials and Supplies Materials and supplies represent consumables, small spare parts and construction materials held for internal construction and maintenance of property, plant and equipment. These assets are carried at average cost less any impairments recorded.
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment is recorded at original cost, net of customer contributions, and any accumulated impairment losses. The cost of additions, including betterments and replacement asset components, is included on the Consolidated Balance Sheets as property, plant and equipment. The original cost of property, plant and equipment includes direct materials, direct labour (including employee benefits), contracted services, attributable capitalized financing costs, asset retirement costs, and direct and indirect overheads that are related to the capital project or program. Indirect overheads include a portion of corporate costs such as finance, treasury, human resources, information technology and executive costs. Overhead costs, including corporate functions and field services costs, are capitalized on a fully allocated basis, consistent with an OEB-approved methodology. Property, plant and equipment in service consists of transmission, distribution, communication, administration and service assets and land easements. Property, plant and equipment also includes future use assets, such as land, major components and spare parts, and capitalized project development costs associated with deferred capital projects. Transmission Transmission assets include assets used for the transmission of high-voltage electricity, such as transmission lines, support structures, foundations, insulators, connecting hardware and grounding systems, and assets used to step up the voltage of electricity from generating stations for transmission and to step down voltages for distribution, including transformers, circuit breakers and switches. Distribution Distribution assets include assets related to the distribution of low-voltage electricity, including lines, poles, switches, transformers, protective devices and metering systems. Communication Communication assets include fibre optic and microwave radio systems, optical ground wire, towers, telephone equipment and associated buildings. Administration and Service Administration and service assets include administrative buildings, personal computers, transport and work equipment, tools and other minor assets. Easements Easements include statutory rights of use for transmission corridors and abutting lands granted under the Reliable Energy and Consumer Protection Act, 2002
Intangible Assets	Intangible Assets Intangible assets separately acquired or internally developed are measured on initial recognition at cost, which comprises purchased software, direct labour (including employee benefits), consulting, engineering, overheads and attributable capitalized financing charges. Following initial recognition, intangible assets are carried at cost, net of any accumulated amortization and accumulated impairment losses. The Company’s intangible assets primarily represent major computer applications.
Capitalized Financing Costs	Capitalized Financing Costs Capitalized financing costs represent interest costs attributable to the construction of property, plant and equipment or development of intangible assets. The financing cost of attributable borrowed funds is capitalized as part of the acquisition cost of such assets. The capitalized financing costs are a reduction of financing charges recognized in the Consolidated Statements of Operations and Comprehensive Income. Capitalized financing costs are calculated using the Company’s weighted average effective cost of debt.
Construction and Development in Progress	Construction and Development in Progress Construction and development in progress consists of the capitalized cost of constructed assets that are not yet complete and which have not yet been placed in service.
Depreciation and Amortization	Depreciation and Amortization The cost of property, plant and equipment and intangible assets is depreciated or amortized on a straight-line basis based on the estimated remaining service life of each asset category, except for transport and work equipment, which is depreciated on a declining balance basis. The Company periodically initiates an external independent review of its property, plant and equipment and intangible asset depreciation and amortization rates, as required by the OEB. Any changes arising from OEB approval of such a review are implemented on a remaining service life basis, consistent with their inclusion in electricity rates. The last review resulted in changes to rates effective January 1, 2015. A summary of average service lives and depreciation and amortization rates for the various classes of assets is included below: Average Rate Service Life Range Average Property, plant and equipment: Transmission 56 years 1% – 3 % 2 % Distribution 46 years 1% – 7 % 2 % Communication 16 years 1% – 15 % 6 % Administration and service 18 years 1% – 20 % 7 % Intangible assets 10 years 10 % 10 % In accordance with group depreciation practices, the original cost of property, plant and equipment, or major components thereof, and intangible assets that are normally retired, is charged to accumulated depreciation, with no gain or loss being reflected in results of operations. Where a disposition of property, plant and equipment occurs through sale, a gain or loss is calculated based on proceeds and such gain or loss is included in depreciation expense.
Acquisitions and Goodwill	Acquisitions and Goodwill The Company accounts for business acquisitions using the acquisition method of accounting and, accordingly, the assets and liabilities of the acquired entities are primarily measured at their estimated fair value at the date of acquisition. Goodwill represents the cost of acquired companies that is in excess of the fair value of the net identifiable assets acquired at the acquisition date. Goodwill is not included in rate base. Goodwill is evaluated for impairment on an annual basis, or more frequently if circumstances require. The Company performs a qualitative assessment to determine whether it is more-likely-than-not that the fair value of the applicable reporting unit is less than its carrying amount. If the Company determines, as a result of its qualitative assessment, that it is not more-likely-than-not that the fair value of the applicable reporting unit is less than its carrying amount, no further testing is required. If the Company determines, as a result of its qualitative assessment, that it is more-likely-than-not that the fair value of the applicable reporting unit is less than its carrying amount, a goodwill impairment assessment is performed using a two-step, fair value-based test. The first step compares the fair value of the applicable reporting unit to its carrying amount, including goodwill. If the carrying amount of the applicable reporting unit exceeds its fair value, a second step is performed. The second step requires an allocation of fair value to the individual assets and liabilities using purchase price allocation in order to determine the implied fair value of goodwill. If the implied fair value of goodwill is less than the carrying amount, an impairment loss is recorded as a reduction to goodwill and as a charge to results of operations. For the year ended December 31, 2016, based on the qualitative assessment performed as at September 30, 2016, the Company has determined that it is not more-likely-than-not that the fair value of each applicable reporting unit assessed is less than its carrying amount. As a result, no further testing was performed, and the Company has concluded that goodwill was not impaired at December 31, 2016.
Long-Lived Asset Impairment	Long-Lived Asset Impairment When circumstances indicate the carrying value of long-lived assets may not be recoverable, the Company evaluates whether the carrying value of such assets, excluding goodwill, has been impaired. For such long-lived assets, the Company evaluates whether impairment may exist by estimating future estimated undiscounted cash flows expected to result from the use and eventual disposition of the asset. When alternative courses of action to recover the carrying amount of a long-lived asset are under consideration, a probability-weighted approach is used to develop estimates of future undiscounted cash flows. If the carrying value of the long-lived asset is not recoverable based on the estimated future undiscounted cash flows, an impairment loss is recorded, measured as the excess of the carrying value of the asset over its fair value. As a result, the asset’s carrying value is adjusted to its estimated fair value. Within its regulated business, the carrying costs of most of Hydro One’s long-lived assets are included in rate base where they earn an OEB-approved rate of return. Asset carrying values and the related return are recovered through approved rates. As a result, such assets are only tested for impairment in the event that the OEB disallows recovery, in whole or in part, or if such a disallowance is judged to be probable. As at December 31, 2016 and 2015, no asset impairment had been recorded.
Costs of Arranging Debt Financing	Costs of Arranging Debt Financing For financial liabilities classified as other than held-for-trading, the Company defers the external transaction costs related to obtaining debt financing and presents such amounts net of related debt on the Consolidated Balance Sheets. Deferred debt issuance costs are amortized over the contractual life of the related debt on an effective-interest basis and the amortization is included within financing charges in the Consolidated Statements of Operations and Comprehensive Income. Transaction costs for items classified as held-for-trading are expensed immediately.
Comprehensive Income	Comprehensive Income Comprehensive income is comprised of net income and other comprehensive income (OCI). Hydro One presents net income and OCI in a single continuous Consolidated Statement of Operations and Comprehensive Income.
Financial Assets and Liabilities	Financial Assets and Liabilities All financial assets and liabilities are classified into one of the following five categories: held-to-maturity; loans and receivables; held-for-trading; other liabilities; or available-for-sale. Financial assets and liabilities classified as held-for-trading are measured at fair value. All other financial assets and liabilities are measured at amortized cost, except accounts receivable and amounts due from related parties, which are measured at the lower of cost or fair value. Accounts receivable and amounts due from related parties are classified as loans and receivables. The Company considers the carrying amounts of accounts receivable and amounts due from related parties to be reasonable estimates of fair value because of the short time to maturity of these instruments. Provisions for impaired accounts receivable are recognized as adjustments to the allowance for doubtful accounts and are recognized when there is objective evidence that the Company will not be able to collect amounts according to the original terms. All financial instrument transactions are recorded at trade date. Derivative instruments are measured at fair value. Gains and losses from fair valuation are included within financing charges in the period in which they arise. The Company determines the classification of its financial assets and liabilities at the date of initial recognition. The Company designates certain of its financial assets and liabilities to be held at fair value, when it is consistent with the Company’s risk management policy disclosed in Note 16 – Fair Value of Financial Instruments and Risk Management.
Derivative Instruments and Hedge Accounting	Derivative Instruments and Hedge Accounting The Company closely monitors the risks associated with changes in interest rates on its operations and, where appropriate, uses various instruments to hedge these risks. Certain of these derivative instruments qualify for hedge accounting and are designated as accounting hedges, while others either do not qualify as hedges or have not been designated as hedges (hereinafter referred to as undesignated contracts) as they are part of economic hedging relationships. The accounting guidance for derivative instruments requires the recognition of all derivative instruments not identified as meeting the normal purchase and sale exemption as either assets or liabilities recorded at fair value on the Consolidated Balance Sheets. For derivative instruments that qualify for hedge accounting, the Company may elect to designate such derivative instruments as either cash flow hedges or fair value hedges. The Company offsets fair value amounts recognized on its Consolidated Balance Sheets related to derivative instruments executed with the same counterparty under the same master netting agreement. For derivative instruments that qualify for hedge accounting and which are designated as cash flow hedges, the effective portion of any gain or loss, net of tax, is reported as a component of accumulated OCI (AOCI) and is reclassified to results of operations in the same period or periods during which the hedged transaction affects results of operations. Any gains or losses on the derivative instrument that represent either hedge ineffectiveness or hedge components excluded from the assessment of effectiveness are recognized in results of operations. For fair value hedges, changes in fair value of both the derivative instrument and the underlying hedged exposure are recognized in the Consolidated Statements of Operations and Comprehensive Income in the current period. The gain or loss on the derivative instrument is included in the same line item as the offsetting gain or loss on the hedged item in the Consolidated Statements of Operations and Comprehensive Income. The changes in fair value of the undesignated derivative instruments are reflected in results of operations. Embedded derivative instruments are separated from their host contracts and are carried at fair value on the Consolidated Balance Sheets when: (a) the economic characteristics and risks of the embedded derivative are not clearly and closely related to the economic characteristics and risks of the host contract; (b) the hybrid instrument is not measured at fair value, with changes in fair value recognized in results of operations each period; and (c) the embedded derivative itself meets the definition of a derivative. The Company does not engage in derivative trading or speculative activities and had no embedded derivatives at December 31, 2016 or 2015. Hydro One periodically develops hedging strategies taking into account risk management objectives. At the inception of a hedging relationship where the Company has elected to apply hedge accounting, Hydro One formally documents the relationship between the hedged item and the hedging instrument, the related risk management objective, the nature of the specific risk exposure being hedged, and the method for assessing the effectiveness of the hedging relationship. The Company also assesses, both at the inception of the hedge and on a quarterly basis, whether the hedging instruments are effective in offsetting changes in fair values or cash flows of the hedged items.
Employee Future Benefits	Employee Future Benefits Employee future benefits provided by Hydro One include pension, post-retirement and post-employment benefits. The costs of the Company’s pension, post-retirement and post-employment benefit plans are recorded over the periods during which employees render service. The Company recognizes the funded status of its defined benefit pension, post-retirement and post-employment plans on its Consolidated Balance Sheets and subsequently recognizes the changes in funded status at the end of each reporting year. Defined benefit pension, post-retirement and post-employment plans are considered to be underfunded when the projected benefit obligation exceeds the fair value of the plan assets. Liabilities are recognized on the Consolidated Balance Sheets for any net underfunded projected benefit obligation. The net underfunded projected benefit obligation may be disclosed as a current liability, long-term liability, or both. The current portion is the amount by which the actuarial present value of benefits included in the benefit obligation payable in the next 12 months exceeds the fair value of plan assets. If the fair value of plan assets exceeds the projected benefit obligation of the plan, an asset is recognized equal to the net overfunded projected benefit obligation. The post-retirement and post-employment benefit plans are unfunded because there are no related plan assets. Hydro One recognizes its contributions to the defined contribution pension plan as pension expense, with a portion being capitalized as part of labour costs included in capital expenditures. The expensed amount is included in operation, maintenance and administration costs in the Consolidated Statements of Operations and Comprehensive Income. Defined Benefit Pension Defined benefit pension costs are recorded on an accrual basis for financial reporting purposes. Pension costs are actuarially determined using the projected benefit method prorated on service and are based on assumptions that reflect management’s best estimate of the effect of future events, including future compensation increases. Past service costs from plan amendments and all actuarial gains and losses are amortized on a straight-line basis over the expected average remaining service period of active employees in the plan, and over the estimated remaining life expectancy of inactive employees in the plan. Pension plan assets, consisting primarily of listed equity securities as well as corporate and government debt securities, are fair valued at the end of each year. Hydro One records a regulatory asset equal to the net underfunded projected benefit obligation for its pension plan. Post-retirement and Post-employment Benefits Post-retirement and post-employment benefits are recorded and included in rates on an accrual basis. Costs are determined by independent actuaries using the projected benefit method prorated on service and based on assumptions that reflect management’s best estimates. Past service costs from plan amendments are amortized to results of operations based on the expected average remaining service period. For post-retirement benefits, all actuarial gains or losses are deferred using the “corridor” approach. The amount calculated above the “corridor” is amortized to results of operations on a straight-line basis over the expected average remaining service life of active employees in the plan and over the remaining life expectancy of inactive employees in the plan. The post-retirement benefit obligation is remeasured to its fair value at each year end based on an annual actuarial report, with an offset to the associated regulatory asset, to the extent of the remeasurement adjustment. For post-employment obligations, the associated regulatory liabilities representing actuarial gains on transition to US GAAP are amortized to results of operations based on the “corridor” approach. The actuarial gains and losses on post-employment obligations that are incurred during the year are recognized immediately to results of operations. The post-employment benefit obligation is remeasured to its fair value at each year end based on an annual actuarial report, with an offset to the associated regulatory asset, to the extent of the remeasurement adjustment. All post-retirement and post-employment future benefit costs are attributed to labour and are either charged to results of operations or capitalized as part of the cost of property, plant and equipment and intangible assets.
Stock-Based Compensation	Stock-Based Compensation Share Grant Plans Hydro One measures share grant plans based on fair value of share grants as estimated based on the grant date Hydro One Limited’s share price. The costs are recognized in the financial statements using the graded-vesting attribution method for share grant plans that have both a performance condition and a service condition. The Company records a regulatory asset equal to the accrued costs of share grant plans recognized in each period. Forfeitures are recognized as they occur (see note 3). Directors’ Deferred Share Unit (DSU) Plan The Company records the liabilities associated with its Directors’ DSU Plan at fair value at each reporting date until settlement, recognizing compensation expense over the vesting period on a straight-line basis. The fair value of the DSU liability is based on Hydro One Limited’s common share closing price at the end of each reporting period. Long-term Incentive Plan (LTIP) The Company measures its LTIP at fair value based on the grant date share price of Hydro One Limited’s common shares. The related compensation expense of Hydro One is recognized over the vesting period on a straight-line basis. Forfeitures are recognized as they occur.
Loss Contingencies	Loss Contingencies Hydro One is involved in certain legal and environmental matters that arise in the normal course of business. In the preparation of its Consolidated Financial Statements, management makes judgments regarding the future outcome of contingent events and records a loss for a contingency based on its best estimate when it is determined that such loss is probable and the amount of the loss can be reasonably estimated. Where the loss amount is recoverable in future rates, a regulatory asset is also recorded. When a range estimate for the probable loss exists and no amount within the range is a better estimate than any other amount, the Company records a loss at the minimum amount within the range. Management regularly reviews current information available to determine whether recorded provisions should be adjusted and whether new provisions are required. Estimating probable losses may require analysis of multiple forecasts and scenarios that often depend on judgments about potential actions by third parties, such as federal, provincial and local courts or regulators. Contingent liabilities are often resolved over long periods of time. Amounts recorded in the Consolidated Financial Statements may differ from the actual outcome once the contingency is resolved. Such differences could have a material impact on future results of operations, financial position and cash flows of the Company. Provisions are based upon current estimates and are subject to greater uncertainty where the projection period is lengthy. A significant upward or downward trend in the number of claims filed, the nature of the alleged injuries, and the average cost of resolving each claim could change the estimated provision, as could any substantial adverse or favourable verdict at trial. A federal or provincial legislative outcome or structured settlement could also change the estimated liability. Legal fees are expensed as incurred.
Environmental Liabilities	Environmental Liabilities Environmental liabilities are recorded in respect of past contamination when it is determined that future environmental remediation expenditures are probable under existing statute or regulation and the amount of the future expenditures can be reasonably estimated. Hydro One records a liability for the estimated future expenditures associated with contaminated land assessment and remediation and for the phase-out and destruction of polychlorinated biphenyl (PCB)-contaminated mineral oil removed from electrical equipment, based on the present value of these estimated future expenditures. The Company determines the present value with a discount rate equal to its credit-adjusted risk-free interest rate on financial instruments with comparable maturities to the pattern of future environmental expenditures. As the Company anticipates that the future expenditures will continue to be recoverable in future rates, an offsetting regulatory asset has been recorded to reflect the future recovery of these environmental expenditures from customers. Hydro One reviews its estimates of future environmental expenditures annually, or more frequently if there are indications that circumstances have changed.
Asset Retirement Obligations	Asset Retirement Obligations Asset retirement obligations are recorded for legal obligations associated with the future removal and disposal of long-lived assets. Such obligations may result from the acquisition, construction, development and/or normal use of the asset. Conditional asset retirement obligations are recorded when there is a legal obligation to perform a future asset retirement activity but where the timing and/or method of settlement are conditional on a future event that may or may not be within the control of the Company. In such a case, the obligation to perform the asset retirement activity is unconditional even though uncertainty exists about the timing and/or method of settlement. When recording an asset retirement obligation, the present value of the estimated future expenditures required to complete the asset retirement activity is recorded in the period in which the obligation is incurred, if a reasonable estimate can be made. In general, the present value of the estimated future expenditures is added to the carrying amount of the associated asset and the resulting asset retirement cost is depreciated over the estimated useful life of the asset. Where an asset is no longer in service when an asset retirement obligation is recorded, the asset retirement cost is recorded in results of operations. Some of the Company’s transmission and distribution assets, particularly those located on unowned easements and rights-of-way, may have asset retirement obligations, conditional or otherwise. The majority of the Company’s easements and rights-of-way are either of perpetual duration or are automatically renewed annually. Land rights with finite terms are generally subject to extension or renewal. As the Company expects to use the majority of its facilities in perpetuity, no asset retirement obligations have been recorded for these assets. If, at some future date, a particular facility is shown not to meet the perpetuity assumption, it will be reviewed to determine whether an estimable asset retirement obligation exists. In such a case, an asset retirement obligation would be recorded at that time. The Company’s asset retirement obligations recorded to date relate to estimated future expenditures associated with the removal and disposal of asbestos-containing materials installed in some of its facilities and with the decommissioning of specific switching stations located on unowned sites.
New Accounting Pronouncements	The following tables present Accounting Standards Updates (ASUs) issued by the Financial Accounting Standards Board (FASB) that are applicable to Hydro One: Recently Adopted Accounting Guidance ASU Date issued Description Effective date Impact on Hydro One 2014-16 November 2014 This update clarifies that all relevant terms and features should be considered in evaluating the nature of a host contract for hybrid financial instruments issued in the form of a share. The nature of the host contract depends upon the economic characteristics and risks of the entire hybrid financial instrument. January 1, 2016 No material impact upon adoption 2015-01 January 2015 Extraordinary items are no longer required to be presented separately in the income statement. January 1, 2016 No material impact upon adoption 2015-02 February 2015 Guidance on analysis to be performed to determine whether certain types of legal entities should be consolidated. January 1, 2016 No material impact upon adoption 2015-03 April 2015 Debt issuance costs are required to be presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability consistent with debt discounts or premiums. January 1, 2016 Reclassification of deferred debt issuance costs and net unamortized debt premiums as an offset to long-term debt. Applied retrospectively (see note 15). 2015-05 April 2015 Cloud computing arrangements that have been assessed to contain a software licence should be accounted for as internal-use software. January 1, 2016 No material impact upon adoption 2015-16 September 2015 Adjustments to provisional amounts that are identified during the measurement period of a business combination in the reporting period in which the adjustment amount is determined are required to be recognized. The amount recorded in current period earnings are required to be presented separately on the face of the income statement or disclosed in the notes by line item. January 1, 2016 No material impact upon adoption 2015-17 November 2015 All deferred tax assets and liabilities are required to be classified as noncurrent on the balance sheet. January 1, 2017 This ASU was early adopted as of April 1, 2016 and was applied prospectively. As a result, the current portions of the Company’s deferred income tax assets are reclassified as noncurrent assets on the consolidated Balance Sheet. Prior periods were not retrospectively adjusted (see note 7). 2016-09 March 2016 Several aspects of the accounting for share-based payment transactions were simplified, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. January 1, 2017 This ASU was early adopted as of October 1, 2016 and was applied retrospectively. As a result, the Company accounts for forfeitures as they occur. There were no other material impacts upon adoption. Recently Issued Accounting Guidance Not Yet Adopted ASU Date issued Description Effective date Anticipated impact on Hydro One 2014-09 2015-14 2016-08 2016-10 2016-12 2016-20 May 2014 – December 2016 ASU 2014-09 was issued in May 2014 and provides guidance on revenue recognition relating to the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods and services. ASU 2015-14 deferred the effective date of ASU 2014-09 by one year. Additional ASUs were issued in 2016 that simplify transition and provide clarity on certain aspects of the new standard. January 1, 2018 Hydro One has completed its initial assessment and has identified relevant revenue streams. No quantitative determination has been made as a detailed assessment is now underway and will continue through to the third quarter of 2017, with the end result being a determination of the financial impact of this standard. The Company is on track for implementation of this standard by the effective date. 2016-01 January 2016 This update requires equity investments to be measured at fair value with changes in fair value recognized in net income, and requires enhanced disclosures and presentation of financial assets and liabilities in the financial statements. This ASU also simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment. January 1, 2018 Under assessment 2016-02 February 2016 Lessees are required to recognize the rights and obligations resulting from operating leases as assets (right to use the underlying asset for the term of the lease) and liabilities (obligation to make future lease payments) on the balance sheet. January 1, 2019 An initial assessment is currently underway encompassing a review of all existing leases, which will be followed by a detailed review of relevant contracts. No quantitative determination has been made at this time. The Company is on track for implementation of this standard by the effective date. 2016-05 March 2016 The amendments clarify that a change in the counterparty to a derivative instrument that has been designated as the hedging instrument under Topic 815 does not, in and of itself, require de-designation of that hedging relationship provided that all other hedge accounting criteria continue to be met. January 1, 2018 Under assessment 2016-06 March 2016 Contingent call (put) options that are assessed to accelerate the payment of principal on debt instruments need to meet the criteria of being “clearly and closely related” to their debt hosts. January 1, 2017 No material impact 2016-07 March 2016 The requirement to retroactively adopt the equity method of accounting if an investment qualifies for use of the equity method as a result of an increase in the level of ownership or degree of influence has been eliminated. January 1, 2017 No material impact 2016-11 May 2016 This amendment covers the SEC Staff’s rescinding of certain SEC Staff observer comments that are codified in Topic 605 and Topic 932, effective upon the adoption of Topic 606 and Topic 815, effective to coincide with the effective date of Update 2014-16. January 1, 2019 No material impact 2016-13 June 2016 The amendment provides users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. January 1, 2019 Under assessment 2016-15 August 2016 The amendments provide guidance for eight specific cash flow issues with the objective of reducing the existing diversity in practice. January 1, 2018 Under assessment 2016-16 October 2016 The amendment eliminates the prohibition of recognizing current and deferred income taxes for an intra-entity asset transfer, other than inventory, until the asset has been sold to an outside party. The amendment will permit income tax consequences of such transfers to be recognized when the transfer occurs. January 1, 2018 Under assessment 2016-18 November 2016 The amendment requires that restricted cash or restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning and end-of-period balances in the statement of cash flows. January 1, 2018 Under assessment 2017-01 January 2017 The amendment clarifies the definition of a business and provides additional guidance on evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. January 1, 2018 Under assessment

Significant Accounting Polici40

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Average Service Lives and Depreciation and Amortization Rates	A summary of average service lives and depreciation and amortization rates for the various classes of assets is included below: Average Rate Service Life Range Average Property, plant and equipment: Transmission 56 years 1% – 3 % 2 % Distribution 46 years 1% – 7 % 2 % Communication 16 years 1% – 15 % 6 % Administration and service 18 years 1% – 20 % 7 % Intangible assets 10 years 10 % 10 %

New Accounting Pronouncements (

New Accounting Pronouncements (Tables)	12 Months Ended
	Dec. 31, 2016
Accounting Changes and Error Corrections [Abstract]
Schedule of Accounting Standards Updates Issued by Financial Accounting Standards Board	The following tables present Accounting Standards Updates (ASUs) issued by the Financial Accounting Standards Board (FASB) that are applicable to Hydro One: Recently Adopted Accounting Guidance ASU Date issued Description Effective date Impact on Hydro One 2014-16 November 2014 This update clarifies that all relevant terms and features should be considered in evaluating the nature of a host contract for hybrid financial instruments issued in the form of a share. The nature of the host contract depends upon the economic characteristics and risks of the entire hybrid financial instrument. January 1, 2016 No material impact upon adoption 2015-01 January 2015 Extraordinary items are no longer required to be presented separately in the income statement. January 1, 2016 No material impact upon adoption 2015-02 February 2015 Guidance on analysis to be performed to determine whether certain types of legal entities should be consolidated. January 1, 2016 No material impact upon adoption 2015-03 April 2015 Debt issuance costs are required to be presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability consistent with debt discounts or premiums. January 1, 2016 Reclassification of deferred debt issuance costs and net unamortized debt premiums as an offset to long-term debt. Applied retrospectively (see note 15). 2015-05 April 2015 Cloud computing arrangements that have been assessed to contain a software licence should be accounted for as internal-use software. January 1, 2016 No material impact upon adoption 2015-16 September 2015 Adjustments to provisional amounts that are identified during the measurement period of a business combination in the reporting period in which the adjustment amount is determined are required to be recognized. The amount recorded in current period earnings are required to be presented separately on the face of the income statement or disclosed in the notes by line item. January 1, 2016 No material impact upon adoption 2015-17 November 2015 All deferred tax assets and liabilities are required to be classified as noncurrent on the balance sheet. January 1, 2017 This ASU was early adopted as of April 1, 2016 and was applied prospectively. As a result, the current portions of the Company’s deferred income tax assets are reclassified as noncurrent assets on the consolidated Balance Sheet. Prior periods were not retrospectively adjusted (see note 7). 2016-09 March 2016 Several aspects of the accounting for share-based payment transactions were simplified, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. January 1, 2017 This ASU was early adopted as of October 1, 2016 and was applied retrospectively. As a result, the Company accounts for forfeitures as they occur. There were no other material impacts upon adoption. Recently Issued Accounting Guidance Not Yet Adopted ASU Date issued Description Effective date Anticipated impact on Hydro One 2014-09 2015-14 2016-08 2016-10 2016-12 2016-20 May 2014 – December 2016 ASU 2014-09 was issued in May 2014 and provides guidance on revenue recognition relating to the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods and services. ASU 2015-14 deferred the effective date of ASU 2014-09 by one year. Additional ASUs were issued in 2016 that simplify transition and provide clarity on certain aspects of the new standard. January 1, 2018 Hydro One has completed its initial assessment and has identified relevant revenue streams. No quantitative determination has been made as a detailed assessment is now underway and will continue through to the third quarter of 2017, with the end result being a determination of the financial impact of this standard. The Company is on track for implementation of this standard by the effective date. 2016-01 January 2016 This update requires equity investments to be measured at fair value with changes in fair value recognized in net income, and requires enhanced disclosures and presentation of financial assets and liabilities in the financial statements. This ASU also simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment. January 1, 2018 Under assessment 2016-02 February 2016 Lessees are required to recognize the rights and obligations resulting from operating leases as assets (right to use the underlying asset for the term of the lease) and liabilities (obligation to make future lease payments) on the balance sheet. January 1, 2019 An initial assessment is currently underway encompassing a review of all existing leases, which will be followed by a detailed review of relevant contracts. No quantitative determination has been made at this time. The Company is on track for implementation of this standard by the effective date. 2016-05 March 2016 The amendments clarify that a change in the counterparty to a derivative instrument that has been designated as the hedging instrument under Topic 815 does not, in and of itself, require de-designation of that hedging relationship provided that all other hedge accounting criteria continue to be met. January 1, 2018 Under assessment 2016-06 March 2016 Contingent call (put) options that are assessed to accelerate the payment of principal on debt instruments need to meet the criteria of being “clearly and closely related” to their debt hosts. January 1, 2017 No material impact 2016-07 March 2016 The requirement to retroactively adopt the equity method of accounting if an investment qualifies for use of the equity method as a result of an increase in the level of ownership or degree of influence has been eliminated. January 1, 2017 No material impact 2016-11 May 2016 This amendment covers the SEC Staff’s rescinding of certain SEC Staff observer comments that are codified in Topic 605 and Topic 932, effective upon the adoption of Topic 606 and Topic 815, effective to coincide with the effective date of Update 2014-16. January 1, 2019 No material impact 2016-13 June 2016 The amendment provides users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. January 1, 2019 Under assessment 2016-15 August 2016 The amendments provide guidance for eight specific cash flow issues with the objective of reducing the existing diversity in practice. January 1, 2018 Under assessment 2016-16 October 2016 The amendment eliminates the prohibition of recognizing current and deferred income taxes for an intra-entity asset transfer, other than inventory, until the asset has been sold to an outside party. The amendment will permit income tax consequences of such transfers to be recognized when the transfer occurs. January 1, 2018 Under assessment 2016-18 November 2016 The amendment requires that restricted cash or restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning and end-of-period balances in the statement of cash flows. January 1, 2018 Under assessment 2017-01 January 2017 The amendment clarifies the definition of a business and provides additional guidance on evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. January 1, 2018 Under assessment

Business Combinations (Tables)

Business Combinations (Tables)	12 Months Ended
	Dec. 31, 2016
Great Lakes Power Transmission LP [Member]
Summary of Determination of Fair Value of Assets Acquired and Liabilities Assumed	The following table summarizes the determination of the final fair value of the assets acquired and liabilities assumed: (millions of dollars) Cash and cash equivalents 5 Property, plant and equipment 221 Intangible assets 1 Regulatory assets 50 Goodwill 159 Working capital (2 ) Long-term debt (186 ) Pension and post-employment benefit liabilities, net (5 ) Deferred income taxes (17 ) 226
Acquisition of Woodstock Hydro [Member]
Summary of Determination of Fair Value of Assets Acquired and Liabilities Assumed	The following table summarizes the determination of the fair value of the assets acquired and liabilities assumed: (millions of dollars) Working capital 4 Property, plant and equipment 27 Intangible assets 1 Deferred income tax assets 2 Goodwill 22 Long-term debt (17 ) Derivative instruments (3 ) Post-retirement and post-employment benefit liability (1 ) Regulatory liabilities (1 ) Other long-term liabilities (2 ) 32
Acquisition of Haldimand Hydro [Member]
Summary of Determination of Fair Value of Assets Acquired and Liabilities Assumed	The following table summarizes the determination of the fair value of the assets acquired and liabilities assumed: (millions of dollars) Cash and cash equivalents 3 Working capital 5 Property, plant and equipment 52 Deferred income tax assets 1 Goodwill 33 Long-term debt (18 ) Regulatory liabilities (3 ) 73

Depreciation and Amortization (

Depreciation and Amortization (Tables)	12 Months Ended
	Dec. 31, 2016
Text Block [Abstract]
Schedule of Depreciation and Amortization	Year ended December 31 (millions of dollars) 2016 2015 Depreciation of property, plant and equipment 603 594 Asset removal costs 90 90 Amortization of intangible assets 56 54 Amortization of regulatory assets 20 19 769 757

Financing Charges (Tables)

Financing Charges (Tables)	12 Months Ended
	Dec. 31, 2016
Banking and Thrift, Interest [Abstract]
Schedule of Financing Charges	Year ended December 31 (millions of dollars) 2016 2015 Interest on long-term debt 424 417 Interest on short-term notes 9 2 Other 15 14 Less: Interest capitalized on construction and development in progress (54 ) (52 ) Interest earned on investments (2 ) (3 ) Gain on interest-rate swap agreements — (2 ) 392 376

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Reconciliation between Statutory and Effective Tax Rates	The reconciliation between the statutory and the effective tax rates is provided as follows: Year ended December 31 (millions of dollars) 2016 2015 Income taxes / provision for PILs at statutory rate 231 216 Increase (decrease) resulting from: Net temporary differences recoverable in future rates charged to customers: Capital cost allowance in excess of depreciation and amortization (53 ) (37 ) Pension contributions in excess of pension expense (16 ) (25 ) Overheads capitalized for accounting but deducted for tax purposes (16 ) (15 ) Interest capitalized for accounting but deducted for tax purposes (14 ) (13 ) Environmental expenditures (5 ) (5 ) Other 5 (6 ) Net temporary differences (99 ) (101 ) Net tax benefit resulting from transition from PILs Regime to Federal Tax Regime — (9 ) Hydro One Brampton spin-off — 7 Net permanent differences 3 1 Total income taxes / provision for PILs 135 114
Major Components of Income Tax Expense	The major components of income tax expense are as follows: Year ended December 31 (millions of dollars) 2016 2015 Current income taxes / provision for PILs 24 2,931 Deferred income taxes / provision for (recovery of) PILs 111 (2,817 ) Total income taxes / provision for PILs 135 114 Effective income tax rate 15.5 % 14.0 %
Schedule of Deferred Income Tax Assets and Liabilities	At December 31, 2016 and 2015, deferred income tax assets and liabilities consisted of the following: December 31 (millions of dollars) 2016 2015 Deferred income tax assets Depreciation and amortization in excess of capital cost allowance 477 918 Non-depreciable capital property 271 271 Post-retirement and post-employment benefits expense in excess of cash payments 603 572 Environmental expenditures 74 75 Non-capital losses 213 62 Investment in subsidiaries 75 55 Other 30 2 1,743 1,955 Less: valuation allowance (352 ) (326 ) Total deferred income tax assets 1,391 1,629 Less: current portion — 19 1,391 1,610 December 31 (millions of dollars) 2016 2015 Deferred income tax liabilities Regulatory amounts that are not recognized for tax purposes (153 ) (153 ) Goodwill (10 ) (10 ) Capital cost allowance in excess of depreciation and amortization (64 ) (42 ) Other (11 ) (1 ) Total deferred income tax liabilities (238 ) (206 ) Less: current portion — — (238 ) (206 ) Net deferred income tax assets 1,153 1,423
Major Categories of Net Deferred Income Tax Assets	The net deferred income tax assets are presented on the Consolidated Balance Sheets as follows: December 31 (millions of dollars) 2016 2015 Current: Other current assets — 19 Long-term: Deferred income tax assets 1,213 1,610 Deferred income tax liabilities (60 ) (206 ) Net deferred income tax assets 1,153 1,423
Non Capital Losses Carried Forward to Reduce Future Period Taxable Income	As of December 31, 2016, the Company had non-capital losses carried forward available to reduce future years’ taxable income, which expire as follows: Year of expiry (millions of dollars) 2016 2015 2034 2 2 2035 221 232 2036 579 — Total losses 802 234

Accounts Receivable (Tables)

Accounts Receivable (Tables)	12 Months Ended
	Dec. 31, 2016
Receivables [Abstract]
Schedule of Accounts Receivable	December 31 (millions of dollars) 2016 2015 Accounts receivable – billed 427 374 Accounts receivable – unbilled 441 459 Accounts receivable, gross 868 833 Allowance for doubtful accounts (35 ) (61 ) Accounts receivable, net 833 772
Schedule of Allowance for Doubtful Accounts	The following table shows the movements in the allowance for doubtful accounts for the years ended December 31, 2016 and 2015: Year ended December 31 (millions of dollars) 2016 2015 Allowance for doubtful accounts – January 1 (61 ) (66 ) Write-offs 37 37 Additions to allowance for doubtful accounts (11 ) (32 ) Allowance for doubtful accounts – December 31 (35 ) (61 )

Other Current Assets (Tables)

Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Schedule of Other Current Assets	December 31 (millions of dollars) 2016 2015 Regulatory assets (Note 12) 37 36 Materials and supplies 19 21 Deferred income tax assets (Notes 3, 7) — 19 Prepaid expenses and other assets 41 24 97 100

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Schedule of Property, Plant and Equipment	December 31, 2016 (millions of dollars) Property, Plant and Accumulated Construction in Progress Total Transmission 14,692 4,862 910 10,740 Distribution 9,656 3,305 243 6,594 Communication 1,069 674 9 404 Administration and service 1,632 924 61 769 Easements 628 67 — 561 27,677 9,832 1,223 19,068 December 31, 2015 (millions of dollars) Property, Plant and Accumulated Construction in Progress Total Transmission 13,704 4,621 853 9,936 Distribution 9,205 3,177 238 6,266 Communication 1,006 609 18 415 Administration and service 1,531 848 35 718 Easements 622 64 — 558 26,068 9,319 1,144 17,893

Intangible Assets (Tables)

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets	December 31, 2016 (millions of dollars) Intangible Assets Accumulated Development in Progress Total Computer applications software 621 326 53 348 Other 5 4 — 1 626 330 53 349 December 31, 2015 (millions of dollars) Intangible Assets Accumulated Development in Progress Total Computer applications software 579 270 24 333 Other 7 4 — 3 586 274 24 336

Regulatory Assets and Liabili50

Regulatory Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2016
Regulated Operations [Abstract]
Schedule of Regulatory Assets and Liabilities	Hydro One has recorded the following regulatory assets and liabilities: December 31 (millions of dollars) 2016 2015 Regulatory assets: Deferred income tax regulatory asset 1,587 1,445 Pension benefit regulatory asset 900 952 Post-retirement and post-employment benefits 243 240 Environmental 204 207 Retail settlement variance account 145 110 Debt premium 32 — Share-based compensation 31 10 Distribution system code exemption 10 10 2015-2017 rate rider 7 20 B2M LP start-up costs 5 8 Pension cost variance 4 37 Other 14 12 Total regulatory assets 3,182 3,051 Less: current portion 37 36 3,145 3,015 Regulatory liabilities: Green Energy expenditure variance 69 76 External revenue variance 64 87 CDM deferral variance 54 53 Deferred income tax regulatory liability 4 23 Other 18 16 Total regulatory liabilities 209 255 Less: current portion — 19 209 236

Accounts Payable and Other Cu51

Accounts Payable and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2016
Payables and Accruals [Abstract]
Schedule of Accounts Payable and Other Current Liabilities	December 31 (millions of dollars) 2016 2015 Accounts payable 177 152 Accrued liabilities 651 591 Accrued interest 105 96 Regulatory liabilities (Note 12) — 19 933 858

Other Long-Term Liabilities (Ta

Other Long-Term Liabilities (Tables)	12 Months Ended
	Dec. 31, 2016
Other Liabilities Disclosure [Abstract]
Schedule of Other Long-Term Liabilities	December 31 (millions of dollars) 2016 2015 Post-retirement and post-employment benefit liability (Note 18) 1,628 1,541 Pension benefit liability (Note 18) 900 952 Environmental liabilities (Note 19) 177 185 Due to related parties (Note 26) 26 10 Asset retirement obligations (Note 20) 9 9 Long-term accounts payable and other liabilities 25 17 2,765 2,714

Debt and Credit Agreements (Tab

Debt and Credit Agreements (Tables)	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Schedule of Outstanding Long-Term Debt	The following table presents outstanding long-term debt at December 31, 2016 and 2015: December 31 (millions of dollars) 2016 2015 4.64% Series 10 notes due 2016 — 450 Floating-rate Series 27 notes due 2016 1 — 50 5.18% Series 13 notes due 2017 600 600 2.78% Series 28 notes due 2018 750 750 Floating-rate Series 31 notes due 2019 1 228 228 1.48% Series 37 notes due 2019 2 500 — 4.40% Series 20 notes due 2020 300 300 1.62% Series 33 notes due 2020 2 350 350 1.84% Series 34 notes due 2021 500 — 3.20% Series 25 notes due 2022 600 600 2.77% Series 35 notes due 2026 500 — 7.35% Debentures due 2030 400 400 6.93% Series 2 notes due 2032 500 500 6.35% Series 4 notes due 2034 385 385 5.36% Series 9 notes due 2036 600 600 4.89% Series 12 notes due 2037 400 400 6.03% Series 17 notes due 2039 300 300 5.49% Series 18 notes due 2040 500 500 4.39% Series 23 notes due 2041 300 300 6.59% Series 5 notes due 2043 315 315 4.59% Series 29 notes due 2043 435 435 4.17% Series 32 notes due 2044 350 350 5.00% Series 11 notes due 2046 325 325 3.91% Series 36 notes due 2046 350 — 3.72% Series 38 notes due 2047 450 — 4.00% Series 24 notes due 2051 225 225 3.79% Series 26 notes due 2062 310 310 4.29% Series 30 notes due 2064 50 50 Hydro One Inc. long-term debt 10,523 8,723 6.6% Senior Secured Bonds due 2023 (Face value – $112 million) 144 — 4.6% Note Payable due 2023 (Face value – $36 million) 40 — Great Lakes Power long-term debt 184 — 10,707 8,723 Add: Net unamortized debt premiums 3 15 17 Add: Unrealized mark-to-market loss (gain) 2 (2 ) 1 Less: Deferred debt issuance costs 3 (40 ) (34 ) Total long-term debt 10,680 8,707 1 The interest rates of the floating-rate notes are referenced to the 3-month Canadian dollar bankers’ acceptance rate, plus a margin. 2 The unrealized mark-to-market net gain relates to $50 million of the Series 33 notes due 2020 and $500 million Series 37 notes due 2019 (2015 – loss relates to $50 million of the Series 33 notes due 2020). The unrealized mark-to-market net gain is offset by a $2 million (2015 – $1 million) unrealized mark-to-market net loss (2015 – gain) on the related fixed-to-floating interest-rate swap agreements, which are accounted for as fair value hedges. See note 16 – Fair Value of Financial Instruments and Risk Management for details of fair value hedges. 3 Effective January 1, 2016, deferred debt issuance costs and net unamortized debt premiums were reclassified from other long-term assets and other long-term liabilities, respectively, as an offset to long-term debt upon adoption of ASU 2015-03
Schedule of Long-Term Debt	The total long-term debt is presented on the consolidated balance sheets as follows: December 31 (millions of dollars) 2016 2015 Current liabilities: Long-term debt payable within one year 602 500 Long-term liabilities: Long-term debt 10,078 8,207 Total long-term debt 10,680 8,707
Summary of Principal Repayments and Related Weighted Average Interest Rates	Principal repayments and related weighted average interest rates are summarized by the number of years to maturity in the following table: Long-term Debt Principal Repayments Weighted Average Interest Rate Years to Maturity (millions of dollars) (%) 1 year 602 5.2 2 years 753 2.8 3 years 731 1.4 4 years 653 2.9 5 years 503 1.9 3,242 2.8 6 – 10 years 1,234 3.3 Over 10 years 6,195 5.2 10,671 4.3
Summary of Long Term Debt Interest Payment Obligations	Interest payment obligations related to long-term debt are summarized by year in the following table: Interest Payments Year (millions of dollars) 2017 456 2018 425 2019 402 2020 384 2021 370 2,037 2022-2026 1,703 2027+ 4,405 8,145

Fair Value of Financial Instr54

Fair Value of Financial Instruments and Risk Management (Tables)	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Summary of Fair Values and Carrying Values of Long-Term Debt	The fair values and carrying values of the Company’s long-term debt at December 31, 2016 and 2015 are as follows: December 31 (millions of dollars) 2016 Carrying Value 2016 Fair Value 2015 Carrying Value 2015 Fair Value Long-term debt $50 million of MTN Series 33 notes 50 50 51 51 $500 million of MTN Series 37 notes 498 498 — — Other notes and debentures 10,132 11,462 8,656 9,942 10,680 12,010 8,707 9,993
Summary of Fair Value Hierarchy of Financial Assets and Liabilities	The fair value hierarchy of financial assets and liabilities at December 31, 2016 and 2015 is as follows: December 31, 2016 (millions of dollars) Carrying Value Fair Value Level 1 Level 2 Level 3 Assets: Cash and cash equivalents 48 48 48 — — 48 48 48 — — Liabilities: Short-term notes payable 469 469 469 — — Long-term debt, including current portion 10,680 12,010 — 12,010 — Derivative instruments Fair value hedges – interest-rate swaps 2 2 2 — — 11,151 12,481 471 12,010 — December 31, 2015 (millions of dollars) Carrying Value Fair Value Level 1 Level 2 Level 3 Assets: Cash and cash equivalents 89 89 89 — — Derivative instruments Fair value hedge – interest-rate swap 1 1 1 — — 90 90 90 — — Liabilities: Short-term notes payable 1,491 1,491 1,491 — — Long-term debt, including current portion 8,707 9,993 — 9,993 — 10,198 11,484 1,491 9,993 —

Capital Management (Tables)

Capital Management (Tables)	12 Months Ended
	Dec. 31, 2016
Text Block [Abstract]
Summary of Company's Capital Structure	At December 31, 2016 and 2015, the Company’s capital structure was as follows: December 31 (millions of dollars) 2016 2015 Long-term debt payable within one year 602 500 Short-term notes payable 469 1,491 Less: cash and cash equivalents 48 89 1,023 1,902 Long-term debt 10,078 8,207 Common shares 5,391 6,000 Retained earnings 4,487 3,759 Total capital 20,979 19,868

Pension and Post-Retirement a56

Pension and Post-Retirement and Post-Employment Benefits (Tables)	12 Months Ended
	Dec. 31, 2016
Change in Projected Benefit Obligation and Change in Plan Assets	Pension Benefits Post-Retirement and Post-Employment Benefits Year ended December 31 (millions of dollars) 2016 2015 2016 2015 Change in projected benefit obligation Projected benefit obligation, beginning of year 7,683 7,535 1,591 1,582 Current service cost 144 146 41 43 Employee contributions 45 40 — — Interest cost 308 302 66 64 Benefits paid (354 ) (334 ) (43 ) (47 ) Net actuarial loss (gain) (52 ) (6 ) 14 (27 ) Change due to Hydro One Brampton spin-off — — — (5 ) Change due to Hydro One Telecom spin-off — — — (19 ) Change due to employees transfer — — 7 — Projected benefit obligation, end of year 7,774 7,683 1,676 1,591 Change in plan assets Fair value of plan assets, beginning of year 6,731 6,299 — — Actual return on plan assets 370 582 — — Benefits paid (354 ) (334 ) (43 ) (47 ) Employer contributions 108 177 43 47 Employee contributions 45 40 — — Administrative expenses (26 ) (33 ) — — Fair value of plan assets, end of year 6,874 6,731 — — Unfunded status 900 952 1,676 1,591
Schedule of Benefit Obligations and Plan Assets	Hydro One presents its benefit obligations and plan assets net on its Consolidated Balance Sheets as follows: Pension Benefits Post-Retirement and Post-Employment Benefits December 31 (millions of dollars) 2016 2015 2016 2015 Other assets 1 1 — — — Accrued liabilities — — 55 50 Pension benefit liability 900 952 — — Post-retirement and post-employment benefit liability — — 1,628 2 1,541 Net unfunded status 899 952 1,683 1,591 1 Represents the funded status of Great Lakes Power’s defined benefit pension plan. 2 Includes $7 million (2015 – $nil) relating to Great Lakes Power’s post-employment benefit plans.
Schedule of Projected Benefit Obligation (PBO), Accumulated Benefit Obligation (ABO) and Fair Value of Plan Assets	The following table provides the projected benefit obligation (PBO), accumulated benefit obligation (ABO) and fair value of plan assets for the Pension Plan: December 31 (millions of dollars) 2016 2015 PBO 7,774 7,683 ABO 7,094 7,020 Fair value of plan assets 6,874 6,731
Schedule of Weighted Average Assumptions Used to Determine Benefit Obligations	The following weighted average assumptions were used to determine the benefit obligations at December 31, 2016 and 2015: Pension Benefits Post-Retirement and Post-Employment Benefits Year ended December 31 2016 2015 2016 2015 Significant assumptions: Weighted average discount rate 3.90 % 4.00 % 3.90 % 4.10 % Rate of compensation scale escalation (long-term) 2.50 % 2.50 % 2.50 % 2.50 % Rate of cost of living increase 2.00 % 2.00 % 2.00 % 2.00 % Rate of increase in health care cost trends 1 — — 4.36 % 4.36 % 1 6.25% per annum in 2017, grading down to 4.36% per annum in and after 2031 (2015 – 6.38% in 2016, grading down to 4.36% per annum in and after 2031).
Schedule of Weighted Average Assumptions Used to Determine Net Periodic Benefit Costs	The following weighted average assumptions were used to determine the net periodic benefit costs for the years ended December 31, 2016 and 2015. Assumptions used to determine current year-end benefit obligations are the assumptions used to estimate the subsequent year’s net periodic benefit costs. Year ended December 31 2016 2015 Pension Benefits: Weighted average expected rate of return on plan assets 6.50 % 6.50 % Weighted average discount rate 4.00 % 4.00 % Rate of compensation scale escalation (long-term) 2.50 % 2.50 % Rate of cost of living increase 2.00 % 2.00 % Average remaining service life of employees (years) 15 13 Post-Retirement and Post-Employment Benefits: Weighted average discount rate 4.10 % 4.00 % Rate of compensation scale escalation (long-term) 2.50 % 2.50 % Rate of cost of living increase 2.00 % 2.00 % Average remaining service life of employees (years) 15.3 13.8 Rate of increase in health care cost trends 1 4.36 % 4.36 % 1 6.38% per annum in 2016, grading down to 4.36% per annum in and after 2031 (2015 – 6.52% in 2015, grading down to 4.36% per annum in and after 2031).
Schedule of Effect of One Percent Change in Health Care Cost Trends on Projected Benefit Obligation	The effect of a 1% change in health care cost trends on the projected benefit obligation for the post-retirement and post-employment benefits at December 31, 2016 and 2015 is as follows: December 31 (millions of dollars) 2016 2015 Projected benefit obligation: Effect of a 1% increase in health care cost trends 286 252 Effect of a 1% decrease in health care cost trends (219 ) (196 )
Schedule of Effect of One Percent Change in Health Care Cost Trends on Service Cost and Interest Cost	The effect of a 1% change in health care cost trends on the service cost and interest cost for the post-retirement and post-employment benefits for the years ended December 31, 2016 and 2015 is as follows: Year ended December 31 (millions of dollars) 2016 2015 Service cost and interest cost: Effect of a 1% increase in health care cost trends 22 22 Effect of a 1% decrease in health care cost trends (16 ) (16 )
Approximate Life Expectancies Used to Determine Projected Benefit Obligations for Pension, Post-Retirement and Post-Employment Plans	The following approximate life expectancies were used in the mortality assumptions to determine the projected benefit obligations for the pension and post-retirement and post-employment plans at December 31, 2016 and 2015: December 31, 2016 December 31, 2015 Life expectancy at 65 for a member currently at Life expectancy at 65 for a member currently at Age 65 Age 45 Age 65 Age 45 Male Female Male Female Male Female Male Female 22 24 23 24 23 25 24 26
Schedule of Estimated Future Benefit Payments	At December 31, 2016, estimated future benefit payments to the participants of the Plans were: (millions of dollars) Pension Benefits Post-Retirement and Post-Employment Benefits 2017 321 55 2018 331 57 2019 340 60 2020 349 61 2021 358 65 2022 through to 2026 1,910 353 Total estimated future benefit payments through to 2026 3,609 651
Schedule of Actuarial Gains and Losses and Prior Service Costs Recorded Within Regulatory Assets	The following table provides the actuarial gains and losses and prior service costs recorded within regulatory assets: Year ended December 31 (millions of dollars) 2016 2015 Pension Benefits: Actuarial loss (gain) for the year 35 (181 ) Amortization of actuarial losses (96 ) (119 ) Prior service cost amortization — (2 ) (61 ) (302 ) Post-Retirement and Post-Employment Benefits: Actuarial loss (gain) for the year 14 (27 ) Amortization of actuarial losses (15 ) (14 ) Prior service cost amortization — — (1 ) (41 )
Components of Regulatory Assets That Have Not Been Recognized as Components of Net Periodic Benefit Costs	The following table provides the components of regulatory assets that have not been recognized as components of net periodic benefit costs for the years ended December 31, 2016 and 2015: Year ended December 31 (millions of dollars) 2016 2015 Pension Benefits: Prior service cost — — Actuarial loss 900 952 900 952 Post-Retirement and Post-Employment Benefits: Actuarial loss 243 240 243 240
Components of Regulatory Assets Expected to be Amortized as Components of Net Periodic Benefit Costs	The following table provides the components of regulatory assets at December 31 that are expected to be amortized as components of net periodic benefit costs in the following year: Pension Benefits Post-Retirement and Post-Employment Benefits December 31 (millions of dollars) 2016 2015 2016 2015 Prior service cost — — — — Actuarial loss 79 96 6 8 79 96 6 8
Schedule of Pension Plan Target Asset and Weighted Average Asset Allocations	At December 31, 2016, the Pension Plan target asset allocations and weighted average asset allocations were as follows: Target Allocation (%) Pension Plan Assets (%) Equity securities 55.0 58.7 Debt securities 35.0 33.6 Other 1 10.0 7.7 100.0 100.0 1 Other investments include real estate and infrastructure investments.
Pension Plan Assets Measured and Recorded at Fair Value on Recurring Basis	The following tables present the Pension Plan assets measured and recorded at fair value on a recurring basis and their level within the fair value hierarchy at December 31, 2016 and 2015: December 31, 2016 (millions of dollars) Level 1 Level 2 Level 3 Total Pooled funds — 20 425 445 Cash and cash equivalents 146 — — 146 Short-term securities — 127 — 127 Corporate shares – Canadian 911 — — 911 Corporate shares – Foreign 2,985 113 — 3,098 Bonds and debentures – Canadian — 1,943 — 1,943 Bonds and debentures – Foreign — 193 — 193 Total fair value of plan assets 1 4,042 2,396 425 6,863 1 At December 31, 2016, the total fair value of Pension Plan assets excludes $27 million of interest and dividends receivable, $15 million of purchased investments payable, $9 million of pension administration expenses payable, and $7 million of sold investments receivable. December 31, 2015 (millions of dollars) Level 1 Level 2 Level 3 Total Pooled funds — 23 301 324 Cash and cash equivalents 191 — — 191 Short-term securities — 80 — 80 Corporate shares – Canadian 807 — — 807 Corporate shares – Foreign 2,931 116 — 3,047 Bonds and debentures – Canadian — 2,072 — 2,072 Bonds and debentures – Foreign — 201 — 201 Total fair value of plan assets 1 3,929 2,492 301 6,722 1 At December 31, 2015, the total fair value of Pension Plan assets excludes $27 million of interest and dividends receivable, and $18 million relating to accruals for pension administration expense and foreign exchange contracts payable.
Changes in Fair Value of Financial Instruments Classified in Level 3	The following table summarizes the changes in fair value of financial instruments classified in Level 3 for the years ended December 31, 2016 and 2015. The Pension Plan classifies financial instruments as Level 3 when the fair value is measured based on at least one significant input that is not observable in the markets or due to lack of liquidity in certain markets. The gains and losses presented in the table below may include changes in fair value based on both observable and unobservable inputs. Year ended December 31 (millions of dollars) 2016 2015 Fair value, beginning of year 301 144 Realized and unrealized gains 23 51 Purchases 151 106 Sales and disbursements (50 ) — Fair value, end of year 425 301
Pension Plan [Member]
Components of Net Periodic Benefit Costs	The following table provides the components of the net periodic benefit costs for the years ended December 31, 2016 and 2015 for the Pension Plan: Year ended December 31 (millions of dollars) 2016 2015 Current service cost, net of employee contributions 144 146 Interest cost 308 302 Expected return on plan assets, net of expenses (432 ) (406 ) Amortization of actuarial losses 96 119 Prior service cost amortization — 2 Net periodic benefit costs 116 163 Charged to results of operations 1 45 81 1 The Company follows the cash basis of accounting consistent with the inclusion of pension costs in OEB-approved rates. During the year ended December 31, 2016, pension costs of $105 million (2015 – $177 million) were attributed to labour, of which $45 million (2015 – $81 million) was charged to operations, and $60 million (2015 – $96 million) was capitalized as part of the cost of property, plant and equipment and intangible assets.
Post-Retirement and Post-Employment Benefits [Member]
Components of Net Periodic Benefit Costs	The following table provides the components of the net periodic benefit costs for the years ended December 31, 2016 and 2015 for the post-retirement and post-employment benefit plans: Year ended December 31 (millions of dollars) 2016 2015 Current service cost, net of employee contributions 41 43 Interest cost 66 64 Amortization of actuarial losses 15 14 Prior service cost amortization — — Net periodic benefit costs 122 121 Charged to results of operations 53 55

Environmental Liabilities (Tabl

Environmental Liabilities (Tables)	12 Months Ended
	Dec. 31, 2016
Text Block [Abstract]
Schedule of Movements in Environmental Liabilities	The following tables show the movements in environmental liabilities for the years ended December 31, 2016 and 2015: Year ended December 31, 2016 (millions of dollars) PCB Land Assessment Total Environmental liabilities, January 1 148 59 207 Interest accretion 7 1 8 Expenditures (11 ) (9 ) (20 ) Revaluation adjustment (1 ) 10 9 Environmental liabilities, December 31 143 61 204 Less: current portion 18 9 27 125 52 177 Year ended December 31, 2015 (millions of dollars) PCB Land Assessment Total Environmental liabilities, January 1 172 67 239 Interest accretion 8 2 10 Expenditures (8 ) (11 ) (19 ) Revaluation adjustment (24 ) 1 (23 ) Environmental liabilities, December 31 148 59 207 Less: current portion 12 10 22 136 49 185
Reconciliation between Undiscounted Basis of Environmental Liabilities and Amount Recognized on Consolidated Balance Sheets	The following tables show the reconciliation between the undiscounted basis of the environmental liabilities and the amount recognized on the Consolidated Balance Sheets after factoring in the discount rate: December 31, 2016 (millions of dollars) PCB Land Assessment Total Undiscounted environmental liabilities 158 66 224 Less: discounting accumulated liabilities to present value 15 5 20 Discounted environmental liabilities 143 61 204 December 31, 2015 (millions of dollars) PCB Land Assessment Total Undiscounted environmental liabilities 168 61 229 Less: discounting accumulated liabilities to present value 20 2 22 Discounted environmental liabilities 148 59 207
Schedule of Estimated Future Environmental Expenditures	At December 31, 2016, the estimated future environmental expenditures were as follows: (millions of dollars) 2017 27 2018 26 2019 25 2020 29 2021 36 Thereafter 81 224

Share Capital (Tables)

Share Capital (Tables)	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Number of Common Shares	The following table presents the common shares issued during the year ended December 31, 2015. Year ended December 31, 2015 (millions of dollars) (number of shares) Pre-Closing Transactions: Common shares issued – purchase and cancellation of preferred shares (a) 323 2,640 Common shares issued (b) 2,600 39,598 Common shares issued (c) — 1 Total common shares issued 2,923 42,239 (a) On October 31, 2015, Hydro One purchased and cancelled its 12,920,000 preferred shares previously held by the Province for cancellation at a price equal to the redemption price of the preferred shares totalling $323 million, which was satisfied by the issuance to the Province of 2,640 common shares of Hydro One. (b) On November 4, 2015, Hydro One issued 39,598 common shares to Hydro One Limited for proceeds of $2.6 billion. (c) On November 3, 2015, Hydro One declared a stock dividend on its common shares, which due to the number of shares issued and the resulting effect on the price per share was treated as a stock split. On November 5, 2015, Hydro One effected a reverse split and issued as consideration one common share to Hydro One Limited. There was no impact to the capital structure of Hydro One as a net result of the stock dividend and the reverse split.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Share Grant Activity	A summary of share grant activity under the Share Grant Plans during years ended December 31, 2016 and 2015 is presented below: Year ended December 31, 2016 Share Grants (number of common shares) Weighted-Average Price Share grants outstanding – January 1, 2016 5,319,370 $ 20.50 Granted (non-vested) — — Forfeited 1 (79,692 ) $ 20.50 Share grants outstanding – December 31, 2016 5,239,678 $ 20.50 1 Includes shares forfeited as well as shares transferred corresponding to transfer of employees from an affiliate company. Year ended December 31, 2015 Share Grants (number of common shares) Weighted-Average Price Share grants outstanding – January 1, 2015 — — Granted (non-vested) 5,319,370 $ 20.50 Share grants outstanding – December 31, 2015 5,319,370 $ 20.50
Summary of Number of DSUs	Each DSU represents a unit with an underlying value equivalent to the value of one common share of Hydro One Limited and is entitled to accrue Hydro One Limited common share dividend equivalents in the form of additional DSUs at the time dividends are paid, subsequent to declaration by Hydro One Limited’s Board of Directors. Year ended December 31 (number of DSUs) 2016 2015 DSUs outstanding – January 1 20,525 — DSUs granted 78,558 20,525 DSUs outstanding – December 31 99,083 20,525
Summary of Number of PSUs and RSUs	During 2016, Hydro One Limited granted awards under its LTIP, consisting of PSUs and RSUs, all of which are equity settled in Hydro One Limited shares, as follows: Year ended December 31, 2016 Number of PSUs Number of RSUs Units outstanding – January 1, 2016 — — Units granted 233,710 257,260 Units forfeited (4,820 ) (4,820 ) Units outstanding – December 31, 2016 228,890 252,440

Noncontrolling Interest (Tables

Noncontrolling Interest (Tables)	12 Months Ended
	Dec. 31, 2016
Noncontrolling Interest [Abstract]
Schedule of Movements in Noncontrolling Interest	The following tables show the movements in noncontrolling interest for the years ended December 31, 2016 and 2015: Year ended December 31, 2016 (millions of dollars) Temporary Equity Equity Total Noncontrolling interest – January 1, 2016 23 52 75 Distributions to noncontrolling interest (3 ) (6 ) (9 ) Net income attributable to noncontrolling interest 2 4 6 Noncontrolling interest – December 31, 2016 22 50 72 Year ended December 31, 2015 (millions of dollars) Temporary Equity Equity Total Noncontrolling interest – January 1, 2015 21 49 70 Distributions to noncontrolling interest (1 ) (4 ) (5 ) Net income attributable to noncontrolling interest 3 7 10 Noncontrolling interest – December 31, 2015 23 52 75

Related Party Transactions (Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 2016
Related Party Transactions [Abstract]
Summary of Related Party Transactions	The Province is the majority shareholder of Hydro One Limited. The IESO, Ontario Power Generation Inc. (OPG), OEFC, OEB, Hydro One Brampton and Hydro One Telecom are related parties to Hydro One because they are controlled or significantly influenced by the Province or by Hydro One Limited. Year ended December 31 2016 2015 Related Party Transaction (millions of dollars) Province 1 Dividends paid — 888 IESO Power purchased 2,096 2,318 Revenues for transmission services 1,549 1,548 Distribution revenues related to rural rate protection 125 127 Distribution revenues related to the supply of electricity to remote northern communities 32 32 Funding received related to Conservation and Demand Management programs 63 70 OPG Power purchased 6 11 Revenues related to provision of construction and equipment maintenance services 4 7 Costs expensed related to the purchase of services 1 1 OEFC Payments in lieu of corporate income taxes 2 — 2,933 Power purchased from power contracts administered by the OEFC 1 6 Indemnification fee paid (terminated effective October 31, 2015) — 8 OEB OEB fees 11 12 Hydro One Brampton 1 Revenues from management, administrative and smart meter network services 3 1 Hydro One Limited Common shares issued 3 — 2,600 Return of stated capital 609 — Dividends paid 2 — Stock-based compensation costs 24 10 IPO costs subsequently reimbursed by Hydro One Limited 4 — 7 Hydro One Telecom Services received – costs expensed 24 4 Services received – costs capitalized 12 2 Revenues for services provided 3 — 1 On August 31, 2015, Hydro One completed the spin-off of its subsidiary, Hydro One Brampton, to the Province. 2 In 2015, Hydro One made PILs to the OEFC totalling $2.9 billion, including Departure Tax of $2.6 billion. 3 On November 4, 2015, Hydro One issued 39,598 common shares to Hydro One Limited for proceeds of $2.6 billion. 4 In 2015, Hydro One incurred certain IPO related expenses totalling $7 million, which were subsequently reimbursed to the Company by Hydro One Limited.
Schedule of Amounts Due to and from Related Parties	The amounts due to and from related parties as a result of the transactions referred to above are as follows: December 31 (millions of dollars) 2016 2015 Due from related parties 224 184 Due to related parties 1 (279 ) (142 ) 1 Included in due to related parties at December 31, 2016 are amounts owing to the IESO in respect of power purchases of $143 million (2015 – $134 million).

Consolidated Statements of Ca62

Consolidated Statements of Cash Flows (Tables)	12 Months Ended
	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Schedule of Consolidated Statement of Cash Flows	The changes in non-cash balances related to operations consist of the following: Year ended December 31 (millions of dollars) 2016 2015 Accounts receivable (59 ) 249 Due from related parties (40 ) 40 Materials and supplies 2 2 Prepaid expenses and other assets (17 ) 12 Accounts payable 18 (26 ) Accrued liabilities 52 (27 ) Due to related parties 113 (95 ) Accrued interest 9 (4 ) Long-term accounts payable and other liabilities 6 — Post-retirement and post-employment benefit liability 84 41 168 192 Capital Expenditures The following table reconciles between investments in property, plant and equipment and the amount presented in the Consolidated Statements of Cash Flows after accounting for capitalized depreciation and the net change in related accruals: Year ended December 31 (millions of dollars) 2016 2015 Capital investments in property, plant and equipment (1,624 ) (1,622 ) Capitalized depreciation and net change in accruals included in capital investments in property, plant and equipment 30 28 Capital expenditures – property, plant and equipment (1,594 ) (1,594 ) The following table reconciles between investments in intangible assets and the amount presented in the Consolidated Statements of Cash Flows after accounting for the net change in related accruals: Year ended December 31 (millions of dollars) 2016 2015 Capital investments in intangible assets (67 ) (40 ) Net change in accruals included in capital investments in intangible assets 6 3 Capital expenditures – intangible assets (61 ) (37 ) Supplementary Information Year ended December 31 (millions of dollars) 2016 2015 Net interest paid 418 416 Income taxes / PILs paid 30 2,928

Commitments (Tables)

Commitments (Tables)	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Summary of Commitments Under Leases, Outsourcing and Other Agreements Due	The following table presents a summary of Hydro One’s commitments under leases, outsourcing and other agreements due in the next 5 years and thereafter. December 31, 2016 (millions of dollars) 2017 2018 2019 2020 2021 Thereafter Outsourcing agreements 165 102 94 2 2 9 Long-term software/meter agreement 17 17 16 17 1 5 Operating lease commitments 9 9 4 8 2 2

Segmented Reporting (Tables)

Segmented Reporting (Tables)	12 Months Ended
	Dec. 31, 2016
Segment Reporting [Abstract]
Summary of Segment Information	The accounting policies followed by the segments are the same as those described in the summary of significant accounting policies (see note 2). Year ended December 31, 2016 (millions of dollars) Transmission Distribution Other Consolidated Revenues 1,587 4,915 — 6,502 Purchased power — 3,427 — 3,427 Operation, maintenance and administration 410 613 20 1,043 Depreciation and amortization 390 379 — 769 Income (loss) before financing charges and income taxes 787 496 (20 ) 1,263 Capital investments 988 703 — 1,691 Year ended December 31, 2015 (millions of dollars) Transmission Distribution Other Consolidated Revenues 1,536 4,949 44 6,529 Purchased power — 3,450 — 3,450 Operation, maintenance and administration 415 633 82 1,130 Depreciation and amortization 374 380 3 757 Income (loss) before financing charges and income taxes 747 486 (41 ) 1,192 Capital investments 943 711 8 1,662 Total Assets by Segment: December 31 (millions of dollars) 2016 2015 Transmission 13,083 12,045 Distribution 9,393 9,200 Other 2,834 2,924 Total assets 25,310 24,169

Description of the Business - A

Description of the Business - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Date of incorporation	Dec. 1, 1998

Significant Accounting Polici66

Significant Accounting Policies - Additional Information (Detail) - CAD	May 01, 2016	Dec. 31, 2015	Nov. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Line Items]
Short-term investments original maturity					3 months
Income tax examination, Likelihood of unfavorable settlement					Tax benefits associated with income tax positions taken, or expected to be taken, in a tax return are recorded only when the "more-likely-than-not" recognition threshold is satisfied and are measured at the largest amount of benefit that has a greater than 50% likelihood of being realized upon settlement.
Threshold probability for recognition					50.00%
Asset impairment charges					CAD 0	CAD 0
Embedded derivatives		CAD 0			0	0
Asset retirement obligation		9,000,000			9,000,000	CAD 9,000,000
Facilities In Perpetuity [Member]
Accounting Policies [Line Items]
Asset retirement obligation					0
Hydro One Networks [Member] | 2016 Approved Revenue Requirement [Member]
Accounting Policies [Line Items]
Transmission revenue requirement			CAD 1,480,000,000
Distribution revenue requirement				CAD 1,430,000,000
Hydro One Networks [Member] | 2015 Approved Revenue Requirement [Member]
Accounting Policies [Line Items]
Distribution revenue requirement				1,326,000,000
Hydro One Networks [Member] | 2017 Approved Revenue Requirement [Member]
Accounting Policies [Line Items]
Distribution revenue requirement				CAD 1,486,000,000
Hydro One Networks [Member] | 2016 Subsequently Approved Updated Revenue Requirements [Member]
Accounting Policies [Line Items]
Distribution revenue requirement					1,410,000,000
Hydro One Networks [Member] | 2017 Subsequently Approved Updated Revenue Requirement [Member]
Accounting Policies [Line Items]
Distribution revenue requirement					CAD 1,415,000,000
B2M Limited Partnership [Member]
Accounting Policies [Line Items]
Ownership interest in related party					66.00%
B2M Limited Partnership [Member] | 2016 Approved Revenue Requirement [Member]
Accounting Policies [Line Items]
Transmission revenue requirement		36,000,000
B2M Limited Partnership [Member] | 2015 Approved Revenue Requirement [Member]
Accounting Policies [Line Items]
Transmission revenue requirement		39,000,000
B2M Limited Partnership [Member] | 2017 Approved Revenue Requirement [Member]
Accounting Policies [Line Items]
Transmission revenue requirement		37,000,000
B2M Limited Partnership [Member] | 2018 Approved Revenue Requirement [Member]
Accounting Policies [Line Items]
Transmission revenue requirement		38,000,000
B2M Limited Partnership [Member] | 2019 Approved Revenue Requirement [Member]
Accounting Policies [Line Items]
Transmission revenue requirement		CAD 37,000,000
Hydro One Remote Communities [Member]
Accounting Policies [Line Items]
Increase in basic rate, distribution and generation of electricity	2.10%
Effective date of increase in basic rate, distribution and generation of electricity	May 1, 2016

Significant Accounting Polici67

Significant Accounting Policies - Average Service Lives and Depreciation and Amortization Rates (Detail)	12 Months Ended
	Dec. 31, 2016
Property Plant And Equipment Estimated Useful Lives [Line Items]
Average Service Life, Intangible assets	10 years
Annual rate of amortization, Intangible assets	10.00%
Annual average rate of amortization, Intangible assets	10.00%
Transmission [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Average Service Life, Property, plant and equipment	56 years
Annual average rate of depreciation and amortization, Property, plant and equipment	2.00%
Distribution [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Average Service Life, Property, plant and equipment	46 years
Annual average rate of depreciation and amortization, Property, plant and equipment	2.00%
Communication [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Average Service Life, Property, plant and equipment	16 years
Annual average rate of depreciation and amortization, Property, plant and equipment	6.00%
Administration and Service [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Average Service Life, Property, plant and equipment	18 years
Annual average rate of depreciation and amortization, Property, plant and equipment	7.00%
Minimum [Member] | Transmission [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Annual rate of depreciation and amortization, Property, plant and equipment	1.00%
Minimum [Member] | Distribution [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Annual rate of depreciation and amortization, Property, plant and equipment	1.00%
Minimum [Member] | Communication [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Annual rate of depreciation and amortization, Property, plant and equipment	1.00%
Minimum [Member] | Administration and Service [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Annual rate of depreciation and amortization, Property, plant and equipment	1.00%
Maximum [Member] | Transmission [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Annual rate of depreciation and amortization, Property, plant and equipment	3.00%
Maximum [Member] | Distribution [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Annual rate of depreciation and amortization, Property, plant and equipment	7.00%
Maximum [Member] | Communication [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Annual rate of depreciation and amortization, Property, plant and equipment	15.00%
Maximum [Member] | Administration and Service [Member]
Property Plant And Equipment Estimated Useful Lives [Line Items]
Annual rate of depreciation and amortization, Property, plant and equipment	20.00%

New Accounting Pronouncements -

New Accounting Pronouncements - Schedule of Accounting Standards Updates Issued by Financial Accounting Standards Board (Detail)	12 Months Ended
	Dec. 31, 2016
Recently Adopted Accounting Guidance [Member] | ASU 2014-16 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2014-11
Description	This update clarifies that all relevant terms and features should be considered in evaluating the nature of a host contract for hybrid financial instruments issued in the form of a share. The nature of the host contract depends upon the economic characteristics and risks of the entire hybrid financial instrument.
Effective date	Jan. 1, 2016
Impact / Anticipated impact on Hydro One	No material impact upon adoption
Recently Adopted Accounting Guidance [Member] | ASU 2015-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2015-01
Description	Extraordinary items are no longer required to be presented separately in the income statement.
Effective date	Jan. 1, 2016
Impact / Anticipated impact on Hydro One	No material impact upon adoption
Recently Adopted Accounting Guidance [Member] | ASU 2015-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2015-02
Description	Guidance on analysis to be performed to determine whether certain types of legal entities should be consolidated.
Effective date	Jan. 1, 2016
Impact / Anticipated impact on Hydro One	No material impact upon adoption
Recently Adopted Accounting Guidance [Member] | ASU 2015-03 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2015-04
Description	Debt issuance costs are required to be presented on the balance sheet as a direct deduction from the carrying amount of the related debt liability consistent with debt discounts or premiums.
Effective date	Jan. 1, 2016
Impact / Anticipated impact on Hydro One	Reclassification of deferred debt issuance costs and net unamortized debt premiums as an offset to long-term debt. Applied retrospectively (see note 15).
Recently Adopted Accounting Guidance [Member] | ASU 2015-05 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2015-04
Description	Cloud computing arrangements that have been assessed to contain a software licence should be accounted for as internal-use software.
Effective date	Jan. 1, 2016
Impact / Anticipated impact on Hydro One	No material impact upon adoption
Recently Adopted Accounting Guidance [Member] | ASU 2015-16 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2015-09
Description	Adjustments to provisional amounts that are identified during the measurement period of a business combination in the reporting period in which the adjustment amount is determined are required to be recognized. The amount recorded in current period earnings are required to be presented separately on the face of the income statement or disclosed in the notes by line item.
Effective date	Jan. 1, 2016
Impact / Anticipated impact on Hydro One	No material impact upon adoption
Recently Adopted Accounting Guidance [Member] | ASU 2015-17 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2015-11
Description	All deferred tax assets and liabilities are required to be classified as noncurrent on the balance sheet.
Effective date	Jan. 1, 2017
Impact / Anticipated impact on Hydro One	This ASU was early adopted as of April 1, 2016 and was applied prospectively. As a result, the current portions of the Company's deferred income tax assets are reclassified as noncurrent assets on the consolidated Balance Sheet. Prior periods were not retrospectively adjusted (see note 7).
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2014-09, 2015-14, 2016- 08, 2016-10, 2016-12, 2016-20 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Description	ASU 2014-09 was issued in May 2014 and provides guidance on revenue recognition relating to the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods and services. ASU 2015-14 deferred the effective date of ASU 2014-09 by one year. Additional ASUs were issued in 2016 that simplify transition and provide clarity on certain aspects of the new standard.
Effective date	Jan. 1, 2018
Impact / Anticipated impact on Hydro One	Hydro One has completed its initial assessment and has identified relevant revenue streams. No quantitative determination has been made as a detailed assessment is now underway and will continue through to the third quarter of 2017, with the end result being a determination of the financial impact of this standard. The Company is on track for implementation of this standard by the effective date.
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2014-09, 2015-14, 2016- 08, 2016-10, 2016-12, 2016-20 [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2014-05
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2014-09, 2015-14, 2016- 08, 2016-10, 2016-12, 2016-20 [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-12
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-01
Description	This update requires equity investments to be measured at fair value with changes in fair value recognized in net income, and requires enhanced disclosures and presentation of financial assets and liabilities in the financial statements. This ASU also simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment.
Effective date	Jan. 1, 2018
Impact / Anticipated impact on Hydro One	Under assessment
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-02
Description	Lessees are required to recognize the rights and obligations resulting from operating leases as assets (right to use the underlying asset for the term of the lease) and liabilities (obligation to make future lease payments) on the balance sheet.
Effective date	Jan. 1, 2019
Impact / Anticipated impact on Hydro One	An initial assessment is currently underway encompassing a review of all existing leases, which will be followed by a detailed review of relevant contracts. No quantitative determination has been made at this time. The Company is on track for implementation of this standard by the effective date.
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-05 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-03
Description	The amendments clarifies that a change in the counterparty to a derivative instrument that has been designated as the hedging instrument under Topic 815 does not, in and of itself, require de-designation of that hedging relationship provided that all other hedge accounting criteria continue to be met.
Effective date	Jan. 1, 2018
Impact / Anticipated impact on Hydro One	Under assessment
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-06 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-03
Description	Contingent call (put) options that are assessed to accelerate the payment of principal on debt instruments need to meet the criteria of being "clearly and closely related" to their debt hosts.
Effective date	Jan. 1, 2017
Impact / Anticipated impact on Hydro One	No material impact
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-03
Description	The requirement to retroactively adopt the equity method of accounting if an investment qualifies for use of the equity method as a result of an increase in the level of ownership or degree of influence has been eliminated.
Effective date	Jan. 1, 2017
Impact / Anticipated impact on Hydro One	No material impact
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-03
Description	Several aspects of the accounting for share-based payment transactions were simplified, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows.
Effective date	Jan. 1, 2017
Impact / Anticipated impact on Hydro One	This ASU was early adopted as of October 1, 2016 and was applied retrospectively. As a result, the Company accounts for forfeitures as they occur. There were no other material impacts upon adoption.
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-11 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-05
Description	This amendment covers the SEC Staff's rescinding of certain SEC Staff observer comments that are codified in Topic 605 and Topic 932, effective upon the adoption of Topic 606 and Topic 815, effective to coincide with the effective date of Update 2014-16.
Effective date	Jan. 1, 2019
Impact / Anticipated impact on Hydro One	No material impact
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-06
Description	The amendment provides users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date.
Effective date	Jan. 1, 2019
Impact / Anticipated impact on Hydro One	Under assessment
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-08
Description	The amendments provide guidance for eight specific cash flow issues with the objective of reducing the existing diversity in practice.
Effective date	Jan. 1, 2018
Impact / Anticipated impact on Hydro One	Under assessment
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-16 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-10
Description	The amendment eliminates the prohibition of recognizing current and deferred income taxes for an intra-entity asset transfer, other than inventory, until the asset has been sold to an outside party. The amendment will permit income tax consequences of such transfers to be recognized when the transfer occurs.
Effective date	Jan. 1, 2018
Impact / Anticipated impact on Hydro One	Under assessment
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2016-18 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2016-11
Description	The amendment requires that restricted cash or restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning and end-of-period balances in the statement of cash flows.
Effective date	Jan. 1, 2018
Impact / Anticipated impact on Hydro One	Under assessment
Recently Issued Accounting Guidance Not Yet Adopted [Member] | ASU 2017-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Date issued	2017-01
Description	The amendment clarifies the definition of a business and provides additional guidance on evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses.
Effective date	Jan. 1, 2018
Impact / Anticipated impact on Hydro One	Under assessment

Business Combinations - Additio

Business Combinations - Additional Information (Detail) - CAD	Oct. 31, 2016	Aug. 15, 2016	Nov. 06, 2015	Oct. 31, 2015	Jun. 30, 2015	Aug. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill							CAD 327,000,000	CAD 163,000,000
Return of stated capital							609,000,000	0
Common Shares [Member]
Business Acquisition [Line Items]
Return of stated capital							609,000,000
Hydro One Telecom spin-off [Member]
Business Acquisition [Line Items]
Transfer of common shares			CAD (17,000,000)
Total debt receivable			(21,000,000)
Return of stated capital			38,000,000
Municipal Billing Services Inc [Member]
Business Acquisition [Line Items]
Return of stated capital			CAD (3,000,000)
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff [Member] | Hydro One Brampton Networks Inc. spin-off [Member]
Business Acquisition [Line Items]
Number of common shares subscribed by Hydro One Inc., shares						357
Value of common shares subscribed						CAD 53,000,000
Long-term intercompany debt aggregate principal amount						193,000,000
Accrued interest						3,000,000
Goodwill						60,000,000
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff [Member] | Hydro One Brampton Networks Inc. spin-off [Member] | Common Shares [Member]
Business Acquisition [Line Items]
Return of stated capital						CAD (196,000,000)
Orillia Power Distribution Corporation [Member]
Business Acquisition [Line Items]
Purchase price of acquisition		CAD 41,000,000
Business acquisition, consideration outstanding indebtedness		CAD 15,000,000
Date of acquisition agreement		Aug. 15, 2016
Great Lakes Power Transmission LP [Member]
Business Acquisition [Line Items]
Purchase price of acquisition	CAD 376,000,000
Business acquisition, consideration outstanding indebtedness	CAD 150,000,000
Date of acquisition agreement	Oct. 31, 2016
Goodwill	CAD 159,000,000
Revenues							6,000,000
Great Lakes Power Transmission LP [Member] | Maximum [Member]
Business Acquisition [Line Items]
Net income							1,000,000
Acquisition of Woodstock Hydro [Member]
Business Acquisition [Line Items]
Purchase price of acquisition				CAD 32,000,000
Date of acquisition agreement				Oct. 31, 2015
Goodwill				CAD 22,000,000
Revenues								12,000,000
Net income								2,000,000
Final purchase price payment							CAD 3,000,000
Acquisition of Haldimand Hydro [Member]
Business Acquisition [Line Items]
Purchase price of acquisition					CAD 73,000,000
Date of acquisition agreement					Jun. 30, 2015
Goodwill					CAD 33,000,000
Revenues								32,000,000
Net income								CAD 6,000,000

Business Combinations - Summary

Business Combinations - Summary of Determination of Fair Value of Assets Acquired and Liabilities Assumed (Detail) - CAD CAD in Millions	Dec. 31, 2016	Oct. 31, 2016	Dec. 31, 2015	Oct. 31, 2015	Jun. 30, 2015
Business Acquisition [Line Items]
Goodwill	CAD 327		CAD 163
Great Lakes Power Transmission LP [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		CAD 5
Working capital		(2)
Property, plant and equipment		221
Intangible assets		1
Regulatory assets		50
Goodwill		159
Long-term debt		(186)
Pension and post-employment benefit liabilities, net		(5)
Deferred income taxes		(17)
Total		CAD 226
Acquisition of Woodstock Hydro [Member]
Business Acquisition [Line Items]
Working capital				CAD 4
Property, plant and equipment				27
Intangible assets				1
Deferred income tax assets				2
Goodwill				22
Long-term debt				(17)
Derivative instruments				(3)
Post-retirement and post-employment benefit liability				(1)
Regulatory liabilities				(1)
Other long-term liabilities				(2)
Total				CAD 32
Acquisition of Haldimand Hydro [Member]
Business Acquisition [Line Items]
Cash and cash equivalents					CAD 3
Working capital					5
Property, plant and equipment					52
Deferred income tax assets					1
Goodwill					33
Long-term debt					(18)
Regulatory liabilities					(3)
Total					CAD 73

Depreciation and Amortization -

Depreciation and Amortization - Schedule of Depreciation and Amortization (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Depreciation, Depletion and Amortization [Abstract]
Depreciation of property, plant and equipment	CAD 603	CAD 594
Asset removal costs	90	90
Amortization of intangible assets	56	54
Amortization of regulatory assets	20	19
Total depreciation and amortization	CAD 769	CAD 757

Financing Charges - Schedule of

Financing Charges - Schedule of Financing Charges (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Interest Income And Interest Expense [Abstract]
Interest on long-term debt	CAD 424	CAD 417
Interest on short-term notes	9	2
Other	15	14
Less: Interest capitalized on construction and development in progress	(54)	(52)
Interest earned on investments	(2)	(3)
Gain on interest-rate swap agreements		(2)
Net financing charges	CAD 392	CAD 376

Income Taxes - Reconciliation b

Income Taxes - Reconciliation between Statutory and Effective Tax Rates (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Net temporary differences recoverable in future rates charged to customers:
Total income taxes / provision for PILs	CAD 135	CAD 114
Income Taxes and Payments in Lieu [Member]
Reconciliation Of Statutory Federal Tax Rate [Line Items]
Income taxes / provision for PILs at statutory rate	231	216
Net temporary differences recoverable in future rates charged to customers:
Capital cost allowance in excess of depreciation and amortization	(53)	(37)
Pension contributions in excess of pension expense	(16)	(25)
Overheads capitalized for accounting but deducted for tax purposes	(16)	(15)
Interest capitalized for accounting but deducted for tax purposes	(14)	(13)
Environmental expenditures	(5)	(5)
Other	5	(6)
Net temporary differences	(99)	(101)
Net tax benefit resulting from transition from PILs Regime to Federal Tax Regime		(9)
Net permanent differences	3	1
Total income taxes / provision for PILs	CAD 135	114
Income Taxes and Payments in Lieu [Member] | Hydro One Brampton Networks Inc. spin-off [Member]
Net temporary differences recoverable in future rates charged to customers:
Hydro One Brampton spin-off		CAD 7

Income Taxes - Major Components

Income Taxes - Major Components of Income Tax Expense (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Components Of Income Tax Expense Benefit [Line Items]
Deferred income taxes / provision for (recovery of) PILs	CAD 111	CAD (2,817)
Total income taxes / provision for PILs	135	114
Income Taxes and Payments in Lieu [Member]
Components Of Income Tax Expense Benefit [Line Items]
Current income taxes / provision for PILs	24	2,931
Deferred income taxes / provision for (recovery of) PILs	111	(2,817)
Total income taxes / provision for PILs	CAD 135	CAD 114
Effective income tax rate	15.50%	14.00%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - CAD CAD in Millions	Nov. 04, 2015	Dec. 31, 2015	Dec. 31, 2016
Income Tax Expenses [Line Items]
Current income taxes / provision due from related parties		CAD 184	CAD 224
Current income taxes / provision due from other current assets		100	97
Payments in lieu of tax		2,600
Proceeds from issuance of common stock	CAD 2,600	2,600
Valuation allowance in respect of capital property		326	352
Income Taxes and Payments in Lieu [Member]
Income Tax Expenses [Line Items]
Current provision for PILs		2,600
Deferred recovery for PILs		2,798
Income Taxes and Payments in Lieu [Member] | CRA [Member]
Income Tax Expenses [Line Items]
Current income taxes / provision due from other current assets		1	13
Income Taxes and Payments in Lieu [Member] | OEFC [Member]
Income Tax Expenses [Line Items]
Current income taxes / provision due from related parties		CAD 12	CAD 6

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Income Tax Assets and Liabilities (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax assets
Depreciation and amortization in excess of capital cost allowance	CAD 477	CAD 918
Non-depreciable capital property	271	271
Post-retirement and post-employment benefits expense in excess of cash payments	603	572
Environmental expenditures	74	75
Non-capital losses	213	62
Investment in subsidiaries	75	55
Other	30	2
Deferred income tax assets gross	1,743	1,955
Less: valuation allowance	(352)	(326)
Total deferred income tax assets	1,391	1,629
Total deferred income tax assets	1,391	1,629
Less: current portion		19
Deferred income tax assets, Noncurrent	1,391	1,610
Deferred income tax liabilities
Regulatory amounts that are not recognized for tax purposes	(153)	(153)
Goodwill	(10)	(10)
Capital cost allowance in excess of depreciation and amortization	(64)	(42)
Other	(11)	(1)
Total deferred income tax liabilities	(238)	(206)
Total deferred income tax liabilities	(238)	(206)
Less: current portion	0	0
Deferred income tax liabilities, Noncurrent	(60)	(206)
Net deferred income tax assets	CAD 1,153	CAD 1,423

Income Taxes - Major Categories

Income Taxes - Major Categories of Net Deferred Income Tax Assets (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Current:
Other current assets		CAD 19
Long-term:
Deferred income tax assets	CAD 1,213	1,610
Deferred income tax liabilities	(60)	(206)
Net deferred income tax assets	CAD 1,153	CAD 1,423

Income Taxes - Non Capital Loss

Income Taxes - Non Capital Losses Carried Forward to Reduce Future Period Taxable Income (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Income Tax Assets And Liabilities [Line Items]
Total losses	CAD 802	CAD 234
2034 [Member]
Deferred Income Tax Assets And Liabilities [Line Items]
Total losses	2	2
2035 [Member]
Deferred Income Tax Assets And Liabilities [Line Items]
Total losses	221	CAD 232
2036 [Member]
Deferred Income Tax Assets And Liabilities [Line Items]
Total losses	CAD 579

Accounts Receivable - Schedule

Accounts Receivable - Schedule of Accounts Receivable (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, gross	CAD 868	CAD 833
Allowance for doubtful accounts	(35)	(61)	CAD (66)
Accounts receivable, net	833	772
Accounts Receivable - Billed [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, gross	427	374
Accounts Receivable - Unbilled [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, gross	CAD 441	CAD 459

Accounts Receivable - Schedul80

Accounts Receivable - Schedule of Allowance for Doubtful Accounts (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Allowance for doubtful accounts beginning balance	CAD (61)	CAD (66)
Write-offs	37	37
Additions to allowance for doubtful accounts	(11)	(32)
Allowance for doubtful accounts ending balance	CAD (35)	CAD (61)

Other Current Assets - Schedule

Other Current Assets - Schedule of Other Current Assets (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Regulatory assets (Note 12)	CAD 37	CAD 36
Materials and supplies	19	21
Deferred income tax assets (Notes 3, 7)		19
Prepaid expenses and other assets	41	24
Other current assets	CAD 97	CAD 100

Property, Plant and Equipment -

Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property, Plant Equipment, Gross	CAD 27,677	CAD 26,068
Accumulated Depreciation	9,832	9,319
Construction in progress	1,223	1,144
Property, plant and equipment, Total	19,068	17,893
Transmission [Member]
Property, Plant and Equipment [Line Items]
Property, Plant Equipment, Gross	14,692	13,704
Accumulated Depreciation	4,862	4,621
Construction in progress	910	853
Property, plant and equipment, Total	10,740	9,936
Distribution [Member]
Property, Plant and Equipment [Line Items]
Property, Plant Equipment, Gross	9,656	9,205
Accumulated Depreciation	3,305	3,177
Construction in progress	243	238
Property, plant and equipment, Total	6,594	6,266
Communication [Member]
Property, Plant and Equipment [Line Items]
Property, Plant Equipment, Gross	1,069	1,006
Accumulated Depreciation	674	609
Construction in progress	9	18
Property, plant and equipment, Total	404	415
Administration and Service [Member]
Property, Plant and Equipment [Line Items]
Property, Plant Equipment, Gross	1,632	1,531
Accumulated Depreciation	924	848
Construction in progress	61	35
Property, plant and equipment, Total	769	718
Easements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant Equipment, Gross	628	622
Accumulated Depreciation	67	64
Property, plant and equipment, Total	CAD 561	CAD 558

Property, Plant and Equipment83

Property, Plant and Equipment - Additional Information (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Property Plant and Equipment Useful Life and Values [Abstract]
Financing charges capitalized on property, plant and equipment	CAD 52	CAD 50

Intangible Assets - Schedule of

Intangible Assets - Schedule of Intangible Assets (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	CAD 626	CAD 586
Accumulated Amortization	330	274
Development in Progress	53	24
Total	349	336
Computer Applications Software [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	621	579
Accumulated Amortization	326	270
Development in Progress	53	24
Total	348	333
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	5	7
Accumulated Amortization	4	4
Total	CAD 1	CAD 3

Intangible Assets - Additional

Intangible Assets - Additional Information (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Financing charges related to intangible assets under development	CAD 2	CAD 1
Amortization expense for intangible assets, 2017	54
Amortization expense for intangible assets, 2018	54
Amortization expense for intangible assets, 2019	45
Amortization expense for intangible assets, 2020	27
Amortization expense for intangible assets, 2021	CAD 26

Regulatory Assets and Liabili86

Regulatory Assets and Liabilities - Schedule of Regulatory Assets and Liabilities (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory assets:
Total regulatory assets	CAD 3,182	CAD 3,051
Total regulatory assets	3,182	3,051
Less: current portion	37	36
Regulatory assets	3,145	3,015
Regulatory liabilities:
Total regulatory liabilities	209	255
Total regulatory liabilities	209	255
Less: current portion		19
Regulatory liabilities	209	236
Deferred Income Tax Regulatory Asset [Member]
Regulatory assets:
Total regulatory assets	1,587	1,445
Total regulatory assets	1,587	1,445
Pension Benefit Regulatory Asset [Member]
Regulatory assets:
Total regulatory assets	900	952
Total regulatory assets	900	952
Post-Retirement and Post-Employment Benefits [Member]
Regulatory assets:
Total regulatory assets	243	240
Total regulatory assets	243	240
Environmental [Member]
Regulatory assets:
Total regulatory assets	204	207
Total regulatory assets	204	207
Retail Settlement Variance Account [Member]
Regulatory assets:
Total regulatory assets	145	110
Total regulatory assets	145	110
Debt Premium [Member]
Regulatory assets:
Total regulatory assets	32
Total regulatory assets	32
Share-Based Compensation [Member]
Regulatory assets:
Total regulatory assets	31	10
Total regulatory assets	31	10
Distribution System Code Exemption [Member]
Regulatory assets:
Total regulatory assets	10	10
Total regulatory assets	10	10
2015 - 2017 Rate Rider [Member]
Regulatory assets:
Total regulatory assets	7	20
Total regulatory assets	7	20
B2M LP Start-Up Costs [Member]
Regulatory assets:
Total regulatory assets	5	8
Total regulatory assets	5	8
Pension Cost Variance [Member]
Regulatory assets:
Total regulatory assets	4	37
Total regulatory assets	4	37
Other Regulatory Assets [Member]
Regulatory assets:
Total regulatory assets	14	12
Total regulatory assets	14	12
Green Energy Expenditure Variance [Member]
Regulatory liabilities:
Total regulatory liabilities	69	76
Total regulatory liabilities	69	76
External Revenue Variance [Member]
Regulatory liabilities:
Total regulatory liabilities	64	87
Total regulatory liabilities	64	87
CDM Deferral Variance [Member]
Regulatory liabilities:
Total regulatory liabilities	54	53
Total regulatory liabilities	54	53
Deferred Income Tax Regulatory Liability [Member]
Regulatory liabilities:
Total regulatory liabilities	4	23
Total regulatory liabilities	4	23
Other Regulatory Liabilities [Member]
Regulatory liabilities:
Total regulatory liabilities	18	16
Total regulatory liabilities	CAD 18	CAD 16

Regulatory Assets and Liabili87

Regulatory Assets and Liabilities - Additional Information (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Regulatory Matters [Line Items]
Increase (Decrease) in income tax expense including the impact of a change in enacted tax rates	CAD (104)	CAD (101)
Post-Retirement and Post-Employment Benefits [Member]
Regulatory Matters [Line Items]
Increase (Decrease) in other comprehensive income	3	(33)
Polychlorinated Biphenyl Liability [Member]
Regulatory Matters [Line Items]
Regulatory asset increase (decrease)	(1)	(24)
Land Assessment and Remediation Liability [Member]
Regulatory Matters [Line Items]
Regulatory asset increase (decrease)	10	1
Environmental Expenditure [Member]
Regulatory Matters [Line Items]
Increase (decrease) in operation, maintenance and administration expenses	(9)	23
Increase (decrease) in amortization expense	20	19
Increase (decrease) in financing chargers	(8)	10
Pension Cost Variance [Member]
Regulatory Matters [Line Items]
Increase (decrease) in revenue	(25)	6
Pension Benefit Regulatory Asset [Member]
Regulatory Matters [Line Items]
Increase (Decrease) in other comprehensive income	CAD (52)	(284)
2015 - 2017 Rate Rider [Member]
Regulatory Matters [Line Items]
Disposal period of approved balances for disposition	32 months
Share-Based Compensation [Member]
Regulatory Matters [Line Items]
Increase (decrease) to operation, maintenance, administration and depreciation expenses	CAD (9)	(5)
B2M LP Start-Up Costs [Member]
Regulatory Matters [Line Items]
Approved recovery start up costs		CAD 8
Start up costs recovery period	4 years

Accounts Payable and Other Cu88

Accounts Payable and Other Current Liabilities - Schedule of Accounts Payable and Other Current Liabilities (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Accounts Payable and Accrued Liabilities, Current [Abstract]
Accounts payable	CAD 177	CAD 152
Accrued liabilities	651	591
Accrued interest	105	96
Regulatory liabilities (Note 12)		19
Total	CAD 933	CAD 858

Other Long-Term Liabilities - S

Other Long-Term Liabilities - Schedule of Other Long-Term Liabilities (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Other Liabilities, Noncurrent [Abstract]
Post-retirement and post-employment benefit liability	CAD 1,628	CAD 1,541
Pension benefit liability	900	952
Environmental liabilities	177	185
Due to related parties	26	10
Asset retirement obligations	9	9
Long-term accounts payable and other liabilities	25	17
Total	CAD 2,765	CAD 2,714

Debt and Credit Agreements - Ad

Debt and Credit Agreements - Additional Information (Detail) - CAD	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Aug. 15, 2016
Debt Instrument [Line Items]
Long-term debt	CAD 10,671,000,000
Issuance of long-term debt under MTN Program	2,300,000,000	CAD 350,000,000
Great Lakes Power Transmission LP [Member]
Debt Instrument [Line Items]
Long-term debt	184,000,000
Medium-Term Note [Member]
Debt Instrument [Line Items]
Remaining borrowing capacity	CAD 1,200,000,000
Remaining available for issuance period	2018-01
Issuance of long-term debt under MTN Program	CAD 2,300,000,000	350,000,000
MTN loan repaid and redeemed	502,000,000	550,000,000
Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt	10,523,000,000	CAD 8,723,000,000
Medium Term Note Program [Member] | Maximum [Member]
Debt Instrument [Line Items]
Long-term debt	3,500,000,000
Revolving Stand By Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			CAD 1,500,000,000
Three Year Senior Revolving Term Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			800,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	2,300,000,000		CAD 2,300,000,000
Commercial Paper Program [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	CAD 1,500,000,000
Maturities days of commercial paper	365 days

Debt and Credit Agreements - Sc

Debt and Credit Agreements - Schedule of Outstanding Long-Term Debt (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term debt, Total	CAD 10,671
Long-term debt, Total	10,707	CAD 8,723
Long-term debt, Total	10,707	8,723
Add: Net unamortized debt premiums	15	17
Add: Unrealized mark-to-market loss (gain)	(2)	1
Less: Deferred debt issuance costs	(40)	(34)
Total long-term debt	10,680	8,707
Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	10,523	8,723
Great Lakes Power Transmission LP [Member]
Debt Instrument [Line Items]
Long-term debt, Total	184
4.64% Series 10 Notes Due 2016 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total		450
Floating-rate Series 27 notes due 2016 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total		50
5.18% Series 13 Notes Due 2017 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	600	600
2.78% Series 28 Notes Due 2018 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	750	750
Floating-rate Series 31 notes due 2019 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	228	228
1.48% Series 37 Notes Due 2019 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	500
4.40% Series 20 Notes Due 2020 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	300	300
1.62% Series 33 Notes Due 2020 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	350	350
1.84% Series 34 Notes Due 2021 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	500
3.20% Series 25 Notes Due 2022 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	600	600
2.77% Series 35 Notes Due 2026 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	500
7.35% Debentures Due 2030 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	400	400
6.93% Series 2 Notes Due 2032 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	500	500
6.35% Series 4 Notes Due 2034 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	385	385
5.36% Series 9 Notes Due 2036 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	600	600
4.89% Series 12 Notes Due 2037 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	400	400
6.03% Series 17 Notes Due 2039 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	300	300
5.49% Series 18 Notes Due 2040 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	500	500
4.39% Series 23 Notes Due 2041 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	300	300
6.59% Series 5 Notes Due 2043 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	315	315
4.59% Series 29 Notes Due 2043 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	435	435
4.17% Series 32 Notes Due 2044 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	350	350
5.00% Series 11 Notes Due 2046 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	325	325
3.91% Series 36 Notes Due 2046 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	350
3.72% Series 38 Notes Due 2047 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	450
4.00% Series 24 Notes Due 2051 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	225	225
3.79% Series 26 Notes Due 2062 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	310	310
4.29% Series 30 Notes Due 2064 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Total	50	50
6.6% Senior Secured Bonds due 2023 [Member] | Great Lakes Power Transmission LP [Member]
Debt Instrument [Line Items]
Long-term debt, Total	112	0
Long-term debt, Total	144
4.6% Note Payable due 2023 [Member] | Great Lakes Power Transmission LP [Member]
Debt Instrument [Line Items]
Long-term debt, Total	36	CAD 0
Long-term debt, Total	CAD 40

Debt and Credit Agreements - 92

Debt and Credit Agreements - Schedule of Outstanding Long-Term Debt (Parenthetical) (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term debt, Face Amount	CAD 10,671
Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Face Amount	10,523	CAD 8,723
Interest-Rate Swaps [Member]
Debt Instrument [Line Items]
Unrealized mark-to-market gain (loss)	CAD (2)	CAD 1
4.64% Series 10 Notes Due 2016 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	4.64%	4.64%
Long-term debt, Maturity year	2,016	2,016
Long-term debt, Face Amount		CAD 450
Floating-rate Series 27 notes due 2016 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Maturity year	2,016	2,016
Long-term debt, Face Amount		CAD 50
5.18% Series 13 Notes Due 2017 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	5.18%	5.18%
Long-term debt, Maturity year	2,017	2,017
Long-term debt, Face Amount	CAD 600	CAD 600
2.78% Series 28 Notes Due 2018 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	2.78%	2.78%
Long-term debt, Maturity year	2,018	2,018
Long-term debt, Face Amount	CAD 750	CAD 750
Floating-rate Series 31 notes due 2019 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Maturity year	2,019	2,019
Long-term debt, Face Amount	CAD 228	CAD 228
1.48% Series 37 Notes Due 2019 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	1.48%	1.48%
Long-term debt, Maturity year	2,019	2,019
Long-term debt, Face Amount	CAD 500
1.48% Series 37 Notes Due 2019 [Member] | Medium-Term Note [Member]
Debt Instrument [Line Items]
Hedged portion of debt	CAD 500
4.40% Series 20 Notes Due 2020 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	4.40%	4.40%
Long-term debt, Maturity year	2,020	2,020
Long-term debt, Face Amount	CAD 300	CAD 300
1.62% Series 33 Notes Due 2020 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	1.62%	1.62%
Long-term debt, Maturity year	2,020	2,020
Long-term debt, Face Amount	CAD 350	CAD 350
1.62% Series 33 Notes Due 2020 [Member] | Medium-Term Note [Member]
Debt Instrument [Line Items]
Hedged portion of debt	CAD 50	CAD 50
1.84% Series 34 Notes Due 2021 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	1.84%	1.84%
Long-term debt, Maturity year	2,021	2,021
Long-term debt, Face Amount	CAD 500
3.20% Series 25 Notes Due 2022 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	3.20%	3.20%
Long-term debt, Maturity year	2,022	2,022
Long-term debt, Face Amount	CAD 600	CAD 600
2.77% Series 35 Notes Due 2026 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	2.77%	2.77%
Long-term debt, Maturity year	2,026	2,026
Long-term debt, Face Amount	CAD 500
7.35% Debentures Due 2030 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	7.35%	7.35%
Long-term debt, Maturity year	2,030	2,030
Long-term debt, Face Amount	CAD 400	CAD 400
6.93% Series 2 Notes Due 2032 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	6.93%	6.93%
Long-term debt, Maturity year	2,032	2,032
Long-term debt, Face Amount	CAD 500	CAD 500
6.35% Series 4 Notes Due 2034 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	6.35%	6.35%
Long-term debt, Maturity year	2,034	2,034
Long-term debt, Face Amount	CAD 385	CAD 385
5.36% Series 9 Notes Due 2036 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	5.36%	5.36%
Long-term debt, Maturity year	2,036	2,036
Long-term debt, Face Amount	CAD 600	CAD 600
4.89% Series 12 Notes Due 2037 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	4.89%	4.89%
Long-term debt, Maturity year	2,037	2,037
Long-term debt, Face Amount	CAD 400	CAD 400
6.03% Series 17 Notes Due 2039 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	6.03%	6.03%
Long-term debt, Maturity year	2,039	2,039
Long-term debt, Face Amount	CAD 300	CAD 300
5.49% Series 18 Notes Due 2040 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	5.49%	5.49%
Long-term debt, Maturity year	2,040	2,040
Long-term debt, Face Amount	CAD 500	CAD 500
4.39% Series 23 Notes Due 2041 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	4.39%	4.39%
Long-term debt, Maturity year	2,041	2,041
Long-term debt, Face Amount	CAD 300	CAD 300
6.59% Series 5 Notes Due 2043 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	6.59%	6.59%
Long-term debt, Maturity year	2,043	2,043
Long-term debt, Face Amount	CAD 315	CAD 315
4.59% Series 29 Notes Due 2043 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	4.59%	4.59%
Long-term debt, Maturity year	2,043	2,043
Long-term debt, Face Amount	CAD 435	CAD 435
4.17% Series 32 Notes Due 2044 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	4.17%	4.17%
Long-term debt, Maturity year	2,044	2,044
Long-term debt, Face Amount	CAD 350	CAD 350
5.00% Series 11 Notes Due 2046 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	5.00%	5.00%
Long-term debt, Maturity year	2,046	2,046
Long-term debt, Face Amount	CAD 325	CAD 325
3.91% Series 36 Notes Due 2046 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	3.91%	3.91%
Long-term debt, Maturity year	2,046	2,046
Long-term debt, Face Amount	CAD 350
3.72% Series 38 Notes Due 2047 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	3.72%	3.72%
Long-term debt, Maturity year	2,047	2,047
Long-term debt, Face Amount	CAD 450
4.00% Series 24 Notes Due 2051 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	4.00%	4.00%
Long-term debt, Maturity year	2,051	2,051
Long-term debt, Face Amount	CAD 225	CAD 225
3.79% Series 26 Notes Due 2062 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	3.79%	3.79%
Long-term debt, Maturity year	2,062	2,062
Long-term debt, Face Amount	CAD 310	CAD 310
4.29% Series 30 Notes Due 2064 [Member] | Medium Term Note Program [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	4.29%	4.29%
Long-term debt, Maturity year	2,064	2,064
Long-term debt, Face Amount	CAD 50	CAD 50
6.6% Senior Secured Bonds due 2023 [Member] | Great Lakes Power Transmission LP [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	6.60%	6.60%
Long-term debt, Maturity year	2,023	2,023
Long-term debt, Face Amount	CAD 112	CAD 0
4.6% Note Payable due 2023 [Member] | Great Lakes Power Transmission LP [Member]
Debt Instrument [Line Items]
Long-term debt, Interest rate	4.60%	4.60%
Long-term debt, Maturity year	2,023	2,023
Long-term debt, Face Amount	CAD 36	CAD 0

Debt and Credit Agreements - 93

Debt and Credit Agreements - Schedule of Long-Term Debt (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Current liabilities:
Long-term debt payable within one year	CAD 602	CAD 500
Long-term liabilities:
Long-term debt, noncurrent	10,078	8,207
Total long-term debt	CAD 10,680	CAD 8,707

Debt and Credit Agreements - Su

Debt and Credit Agreements - Summary of Principal Repayments, Interest Payments and Related Weighted Average Interest Rates (Detail) CAD in Millions	Dec. 31, 2016CAD
Debt Instrument [Line Items]
Long-term Debt Principal Repayments, 1 year	CAD 602
Long-term Debt Principal Repayments, 2 years	753
Long-term Debt Principal Repayments, 3 years	731
Long-term Debt Principal Repayments, 4 years	653
Long-term Debt Principal Repayments, 5 years	503
Long-term Debt Principal Repayments during 5 years	3,242
Long-term Debt Principal Repayments, 6 - 10 years	1,234
Long-term Debt Principal Repayments, Over 10 years	6,195
Long-term debt, Total	CAD 10,671
Weighted Average Interest Rate	4.30%
1 Year [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	5.20%
2 Years [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	2.80%
3 Years [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	1.40%
4 Years [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	2.90%
5 Years [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	1.90%
5 Years, Total [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	2.80%
6-10 Years [Member ]
Debt Instrument [Line Items]
Weighted Average Interest Rate	3.30%
Over 10 Years [Member]
Debt Instrument [Line Items]
Weighted Average Interest Rate	5.20%

Debt and Credit Agreements - 95

Debt and Credit Agreements - Summary of Long Term Debt Interest Payments (Detail) CAD in Millions	12 Months Ended
	Dec. 31, 2016CAD
Debt Instrument [Line Items]
Interest Payments	CAD 8,145
1 Year [Member]
Debt Instrument [Line Items]
Interest Payments	456
2 Years [Member]
Debt Instrument [Line Items]
Interest Payments	425
3 Years [Member]
Debt Instrument [Line Items]
Interest Payments	402
4 Years [Member]
Debt Instrument [Line Items]
Interest Payments	384
5 Years [Member]
Debt Instrument [Line Items]
Interest Payments	370
5 Years, Total [Member]
Debt Instrument [Line Items]
Interest Payments	2,037
6-10 Years [Member ]
Debt Instrument [Line Items]
Interest Payments	1,703
Over 10 Years [Member]
Debt Instrument [Line Items]
Interest Payments	CAD 4,405

Fair Value of Financial Instr96

Fair Value of Financial Instruments and Risk Management - Summary of Fair Values and Carrying Values of Long-Term Debt (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Carrying Value	CAD 10,680	CAD 8,707
Fair Value	12,010	9,993
$500 million of MTN Series 37 notes [Member]
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Carrying Value	498
Fair Value	498
$50 Million of MTN Series 33 Notes [Member]
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Carrying Value	50	51
Fair Value	50	51
Other Notes and Debentures [Member]
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Carrying Value	10,132	8,656
Fair Value	CAD 11,462	CAD 9,942

Fair Value of Financial Instr97

Fair Value of Financial Instruments and Risk Management - Fair Value of Financial Instruments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016CADAgreement	Dec. 31, 2015CAD
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Fair value hedge exposure	5.00%	1.00%
Long-term debt	CAD 10,671,000,000
1.62% Series 33 Notes Due 2020 [Member]
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Fixed-to-floating interest-rate swap	CAD 50,000,000
Number of agreements | Agreement	1
Conversion of debt	CAD 50,000,000
Long-term debt	CAD 350,000,000
Debt maturity date	Apr. 30, 2020
1.48% Series 37 Notes Due 2019 [Member]
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Conversion of debt	CAD 500,000,000
Debt maturity date	Nov. 18, 2019
Interest-Rate Swaps [Member]
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Fixed-to-floating interest-rate swap	CAD 550,000,000	CAD 50,000,000
Notional value	0	CAD 0
125 Fixed-to-Floating Interest-Rate Swap [Member] | 1.48% Series 37 Notes Due 2019 [Member]
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Fixed-to-floating interest-rate swap	CAD 125,000,000
125 Fixed-to-Floating Interest-Rate Swap [Member] | 1.48% Series 37 Notes Due 2019 [Member]
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Number of agreements | Agreement	2
250 Fixed-to-Floating Interest-Rate Swap [Member] | 1.48% Series 37 Notes Due 2019 [Member]
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Fixed-to-floating interest-rate swap	CAD 250,000,000
Number of agreements | Agreement	1

Fair Value of Financial Instr98

Fair Value of Financial Instruments and Risk Management - Fair Value Hierarchy of Financial Assets and Liabilities (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets:
Cash and cash equivalents	CAD 48	CAD 89	CAD 100
Liabilities:
Short-term notes payable	469	1,491
Long-term debt, including current portion	10,680	8,707
Total liabilities, Fair value	12,010	9,993
Total liabilities	15,369	14,344
Level 1 [Member]
Assets:
Cash and cash equivalents	48	89
Fair value hedge - interest-rate swap		1
Total assets	48	90
Liabilities:
Short-term notes payable	469	1,491
Fair value hedge - interest-rate swap	2
Total liabilities, Fair value	471	1,491
Level 2 [Member]
Liabilities:
Total liabilities, Fair value	12,010	9,993
Total liabilities, Fair value	12,010	9,993
Carrying Value [Member]
Assets:
Cash and cash equivalents	48	89
Fair value hedge - interest-rate swap		1
Total assets	48	90
Liabilities:
Short-term notes payable	469	1,491
Long-term debt, including current portion	10,680	8,707
Fair value hedge - interest-rate swap	2
Total liabilities	11,151	10,198
Fair Value [Member]
Assets:
Cash and cash equivalents	48	89
Fair value hedge - interest-rate swap		1
Total assets	48	90
Liabilities:
Short-term notes payable	469	1,491
Total liabilities, Fair value	12,010	9,993
Fair value hedge - interest-rate swap	2
Total liabilities, Fair value	CAD 12,481	CAD 11,484

Fair Value of Financial Instr99

Fair Value of Financial Instruments and Risk Management - Risk Management - Additional Information (Detail) - CAD	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Market Risk, Credit Risk And Liquidity Risk [Line Items]
Provision for bad debts	CAD 35,000,000	CAD 61,000,000	CAD 66,000,000
Accounts payable and accrued liabilities	CAD 828,000,000	CAD 743,000,000
Minimum [Member]
Market Risk, Credit Risk And Liquidity Risk [Line Items]
Account receivable, Period	60 days	60 days
Customer Concentration Risk [Member]
Market Risk, Credit Risk And Liquidity Risk [Line Items]
Accounts receivable	CAD 0	CAD 0
Variable Interest Rate [Member]
Market Risk, Credit Risk And Liquidity Risk [Line Items]
Percentage of increase/(decrease) in interest rate	1.00%	1.00%
Impact on net income	CAD 0	CAD 0
Aged More Than 60 Days [Member]
Market Risk, Credit Risk And Liquidity Risk [Line Items]
Account receivable, Percentage	6.00%	6.00%

Capital Management - Summary of

Capital Management - Summary of Company's Capital Structure (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Long-term debt payable within one year	CAD 602	CAD 500
Short-term notes payable	469	1,491
Less: cash and cash equivalents	48	89	CAD 100
Net Long-term debt payable within one year	1,023	1,902
Long-term debt	10,078	8,207
Common shares	5,391	6,000
Retained earnings	4,487	3,759
Total capital	CAD 20,979	CAD 19,868

Capital Management - Additional

Capital Management - Additional Information (Detail)	Dec. 31, 2016
Statement of Financial Position [Abstract]
Permissible limit on debt to total capital percentage	75.00%

Pension and Post-Retirement 102

Pension and Post-Retirement and Post-Employment Benefits - Additional Information (Detail) - CAD CAD in Millions	Jan. 01, 2016	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accrued liabilities		CAD 651	CAD 591
Pension plan average pensionable earnings		3 years
New pension plan average pensionable earnings		5 years
Annual pension plan contributions		CAD 108	CAD 177
Estimated annual pension plan contributions for 2017		105
Estimated annual pension plan contributions for 2018		CAD 102
Percentage of assets to ascertain concentration of credit risk		10.00%
Option One [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of employer matching contribution	4.00%
Option Two [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of employer matching contribution	5.00%
Option Three [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of employer matching contribution	6.00%
ABO [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Funded percentage		97.00%	96.00%
PBO [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Funded percentage		88.00%	88.00%
Corporate Bond Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Pension plan		CAD 11	CAD 9
Maximum [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Contributions by company to the plan		1
Maximum [Member] | Defined Contribution Pension Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accrued liabilities		1
Province Of Ontario [Member] | Debt Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Pension plan		CAD 450	CAD 420

Pension and Post-Retirement 103

Pension and Post-Retirement and Post-Employment Benefits - Change in Projected Benefit Obligation and Change in Plan Assets (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets
Employer contributions	CAD 108	CAD 177
Hydro One Inc. Pension Plan, Pension Plans Defined Benefit [Member]
Change in projected benefit obligation
Projected benefit obligation, beginning of year	7,683	7,535
Current service cost	144	146
Employee contributions	45	40
Interest cost	308	302
Benefits paid	(354)	(334)
Net actuarial loss (gain)	(52)	(6)
Projected benefit obligation, end of year	7,774	7,683
Change in plan assets
Fair value of plan assets, beginning of year	6,731	6,299
Actual return on plan assets	370	582
Benefits paid	(354)	(334)
Employer contributions	108	177
Employee contributions	45	40
Administrative expenses	(26)	(33)
Fair value of plan assets, end of year	6,874	6,731
Net unfunded status	900	952
Hydro One Inc. Other Post-Retirement And Post-Employment Benefits Plan [Member]
Change in projected benefit obligation
Projected benefit obligation, beginning of year	1,591	1,582
Current service cost	41	43
Interest cost	66	64
Benefits paid	(43)	(47)
Net actuarial loss (gain)	14	(27)
Projected benefit obligation, end of year	1,676	1,591
Change in plan assets
Benefits paid	(43)	(47)
Employer contributions	43	47
Net unfunded status	1,676	1,591
Hydro One Inc. Other Post-Retirement And Post-Employment Benefits Plan [Member] | Hydro One Brampton [Member]
Change in projected benefit obligation
Change in projected benefit obligation due to spin-off		(5)
Hydro One Inc. Other Post-Retirement And Post-Employment Benefits Plan [Member] | Hydro One Telecom spin-off [Member]
Change in projected benefit obligation
Change in projected benefit obligation due to spin-off		CAD (19)
Hydro One Inc. Other Post-Retirement And Post-Employment Benefits Plan [Member] | Employees Transfer [Member]
Change in projected benefit obligation
Change due to employees transfer	CAD 7

Pension and Post-Retirement 104

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Benefit Obligations and Plan Assets (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accrued liabilities	CAD 651	CAD 591
Pension benefit liability	900	952
Hydro One Inc. Pension Plan, Pension Plans Defined Benefit [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net unfunded status	900	952
Pension benefit liability	900	952
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net unfunded status	899	952
Pension Benefits [Member] | Great Lakes Power Transmission LP [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Other assets	1
Post-Retirement and Post-Employment Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accrued liabilities	55	50
Post-retirement and post-employment benefit liability	1,628	1,541
Net unfunded status	CAD 1,683	CAD 1,591

Pension and Post-Retirement 105

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Benefit Obligations and Plan Assets (Parenthetical) (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Great Lakes Power Transmission LP [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Post-employment benefit plans	CAD 7	CAD 0

Pension and Post-Retirement 106

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Projected Benefit Obligation (PBO), Accumulated Benefit Obligation (ABO) and Fair Value of Plan Assets (Detail) - Hydro One Inc. Pension Plan, Pension Plans Defined Benefit [Member] - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
PBO	CAD 7,774	CAD 7,683	CAD 7,535
ABO	7,094	7,020
Fair value of plan assets	CAD 6,874	CAD 6,731	CAD 6,299

Pension and Post-Retirement 107

Pension and Post-Retirement and Post-Employment Benefits - Components of Net Periodic Benefit Costs (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Current service cost, net of employee contributions	CAD 144	CAD 146
Interest cost	308	302
Expected return on plan assets, net of expenses	(432)	(406)
Amortization of actuarial losses	96	119
Prior service cost amortization		2
Net periodic benefit costs	116	163
Charged to results of operations	45	81
Post-Retirement and Post-Employment Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Current service cost, net of employee contributions	41	43
Interest cost	66	64
Amortization of actuarial losses	15	14
Net periodic benefit costs	122	121
Charged to results of operations	CAD 53	CAD 55

Pension and Post-Retirement 108

Pension and Post-Retirement and Post-Employment Benefits - Components of Net Periodic Benefit Costs (Parenthetical) (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Pension costs	CAD 105	CAD 177
Pension costs charged to operations	45	81
Pension costs capitalized	CAD 60	CAD 96

Pension and Post-Retirement 109

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Weighted Average Assumptions Used to Determine Benefit Obligations (Detail)	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Weighted average discount rate	3.90%	4.00%
Rate of compensation scale escalation (long-term)	2.50%	2.50%
Rate of cost of living increase	2.00%	2.00%
Post-Retirement and Post-Employment Benefits [Member]
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Weighted average discount rate	3.90%	4.10%
Rate of compensation scale escalation (long-term)	2.50%	2.50%
Rate of cost of living increase	2.00%	2.00%
Rate of increase in health care cost trends	4.36%	4.36%

Pension and Post-Retirement 110

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Weighted Average Assumptions Used to Determine Benefit Obligations (Parenthetical) (Detail) - Maximum [Member]	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Assumed health care cost trend, percentage	6.38%
2017 [Member]
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Assumed health care cost trend, percentage	6.25%
2016 [Member]
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Assumed health care cost trend, percentage		6.38%
Grading Down in and After 2031 [Member]
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Assumed health care cost trend, percentage	4.36%	4.36%

Pension and Post-Retirement 111

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Weighted Average Assumptions Used to Determine Net Periodic Benefit Costs (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Weighted average expected rate of return on plan assets	6.50%	6.50%
Weighted average discount rate	4.00%	4.00%
Rate of compensation scale escalation (long-term)	2.50%	2.50%
Rate of cost of living increase	2.00%	2.00%
Average remaining service life of employees (years)	15 years	13 years
Post-Retirement and Post-Employment Benefits [Member]
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Weighted average discount rate	4.10%	4.00%
Rate of compensation scale escalation (long-term)	2.50%	2.50%
Rate of cost of living increase	2.00%	2.00%
Average remaining service life of employees (years)	15 years 3 months 18 days	13 years 9 months 18 days
Rate of increase in health care cost trends	4.36%	4.36%

Pension and Post-Retirement 112

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Weighted Average Assumptions Used to Determine Net Periodic Benefit Costs (Parenthetical) (Detail) - Maximum [Member]	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Assumed health care cost trend, percentage	6.38%
Grading Down in and After 2031 [Member]
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Assumed health care cost trend, percentage	4.36%	4.36%
2015 [Member]
Schedule Of Weighted Average Assumption Determining Pension Plan And Other Post retirement Benefit Plan [Line Items]
Assumed health care cost trend, percentage		6.52%

Pension and Post-Retirement 113

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Effect of One Percent Change in Health Care Cost Trends on Projected Benefit Obligation (Detail) - Post-Retirement and Post-Employment Benefits [Member] - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Effect Of One Percentage Point Change In Assumed Health Care Cost Trend Rates [Line Items]
Effect of a 1% increase in health care cost trends	CAD 286	CAD 252
Effect of a 1% decrease in health care cost trends	CAD (219)	CAD (196)

Pension and Post-Retirement 114

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Effect of One Percent Change in Health Care Cost Trends on Service Cost and Interest Cost (Detail) - Post-Retirement and Post-Employment Benefits [Member] - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Effect Of One Percentage Point Change In Assumed Health Care Cost Trend Rates [Line Items]
Effect of a 1% increase in health care cost trends	CAD 22	CAD 22
Effect of a 1% decrease in health care cost trends	CAD (16)	CAD (16)

Pension and Post-Retirement 115

Pension and Post-Retirement and Post-Employment Benefits - Approximate Life Expectancies Used to Determine Projected Benefit Obligations for Pension, Post-Retirement and Post-Employment Plans (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Life Expectancy At Age Sixty Five [Member] | Male [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Approximate life expectancy (in years) at particular age	22 years	23 years
Life Expectancy At Age Sixty Five [Member] | Female [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Approximate life expectancy (in years) at particular age	24 years	25 years
Life Expectancy At Age Forty Five [Member] | Male [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Approximate life expectancy (in years) at particular age	23 years	24 years
Life Expectancy At Age Forty Five [Member] | Female [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Approximate life expectancy (in years) at particular age	24 years	26 years

Pension and Post-Retirement 116

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Estimated Future Benefit Payments (Detail) CAD in Millions	Dec. 31, 2016CAD
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
2,017	CAD 321
2,018	331
2,019	340
2,020	349
2,021	358
2022 through to 2026	1,910
Total estimated future benefit payments through to 2026	3,609
Post-Retirement and Post-Employment Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
2,017	55
2,018	57
2,019	60
2,020	61
2,021	65
2022 through to 2026	353
Total estimated future benefit payments through to 2026	CAD 651

Pension and Post-Retirement 117

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Actuarial Gains and Losses and Prior Service Costs Recorded Within Regulatory Assets (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Benefit Regulatory Asset [Member] | Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Actuarial loss (gain) for the year	CAD 14	CAD (27)
Amortization of actuarial losses	(15)	(14)
Total actuarial gains and losses and prior service costs	(1)	(41)
Post Retirement and Employment Benefits Regulatory Assets [Member] | Post-Retirement and Post-Employment Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Actuarial loss (gain) for the year	35	(181)
Amortization of actuarial losses	(96)	(119)
Prior service cost amortization		(2)
Total actuarial gains and losses and prior service costs	CAD (61)	CAD (302)

Pension and Post-Retirement 118

Pension and Post-Retirement and Post-Employment Benefits - Components of Regulatory Assets That Have Not Been Recognized as Components of Net Periodic Benefit Costs (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Prior service cost	CAD 0	CAD 0
Actuarial loss	900	952
Total	900	952
Post-Retirement and Post-Employment Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Actuarial loss	243	240
Total	CAD 243	CAD 240

Pension and Post-Retirement 119

Pension and Post-Retirement and Post-Employment Benefits - Components of Regulatory Assets Expected to be Amortized as Components of Net Periodic Benefit Costs (Detail) - Regulatory Assets [Member] - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Schedule Of Components Of Regulatory Assets Expected To Be Amortized As Components Of Net Periodic Benefit Cost [Line Items]
Prior service cost	CAD 0	CAD 0
Actuarial loss	79	96
Net periodic benefit costs to be amortized from regulatory assets	79	96
Post-Retirement and Post-Employment Benefits [Member]
Schedule Of Components Of Regulatory Assets Expected To Be Amortized As Components Of Net Periodic Benefit Cost [Line Items]
Prior service cost	0	0
Actuarial loss	6	8
Net periodic benefit costs to be amortized from regulatory assets	CAD 6	CAD 8

Pension and Post-Retirement 120

Pension and Post-Retirement and Post-Employment Benefits - Schedule of Pension Plan Target Asset and Weighted Average Asset Allocations (Detail)	Dec. 31, 2016
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Target Allocation	100.00%
Pension Plan Assets	100.00%
Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Target Allocation	55.00%
Pension Plan Assets	58.70%
Debt Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Target Allocation	35.00%
Pension Plan Assets	33.60%
Other [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Target Allocation	10.00%
Pension Plan Assets	7.70%

Pension and Post-Retirement 121

Pension and Post-Retirement and Post-Employment Benefits - Pension Plan Assets Measured and Recorded at Fair Value on Recurring Basis (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	CAD 6,863	CAD 6,722
Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	445	324
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	146	191
Short-term Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	127	80
Corporate Shares - Canadian [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	911	807
Corporate Shares - Foreign [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	3,098	3,047
Bonds and Debentures - Canadian [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	1,943	2,072
Bonds and debentures - Foreign [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	193	201
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	4,042	3,929
Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	146	191
Level 1 [Member] | Corporate Shares - Canadian [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	911	807
Level 1 [Member] | Corporate Shares - Foreign [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	2,985	2,931
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	2,396	2,492
Level 2 [Member] | Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	20	23
Level 2 [Member] | Short-term Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	127	80
Level 2 [Member] | Corporate Shares - Foreign [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	113	116
Level 2 [Member] | Bonds and Debentures - Canadian [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	1,943	2,072
Level 2 [Member] | Bonds and debentures - Foreign [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	193	201
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	425	301
Level 3 [Member] | Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	CAD 425	CAD 301

Pension and Post-Retirement 122

Pension and Post-Retirement and Post-Employment Benefits - Pension Plan Assets Measured and Recorded at Fair Value on Recurring Basis (Parenthetical) (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Purchase of investments payable excluded from fair value of pension plan assets	CAD 15
Sale of investments receivable excluded from fair value of pension plan assets	7
Interest and dividend receivable excluded from fair value of pension plan assets	27	CAD 27
Accruals for pension administration expense excluded from fair value of pension plan assets	CAD 9	CAD 18

Pension and Post-Retirement 123

Pension and Post-Retirement and Post-Employment Benefits - Changes in Fair Value of Financial Instruments Classified in Level 3 (Detail) - Level 3 [Member] - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value Of Financial Instruments [Line Items]
Fair value of plan assets, beginning of year	CAD 301	CAD 144
Realized and unrealized gains	23	51
Purchases	151	106
Sales and disbursements	(50)
Fair value of plan assets, end of year	CAD 425	CAD 301

Environmental Liabilities - Sch

Environmental Liabilities - Schedule of Movements in Environmental Liabilities (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Environmental Liabilities [Line Items]
Environmental liabilities, Beginning balance	CAD 207	CAD 239
Interest accretion	8	10
Expenditures	(20)	(19)
Revaluation adjustment	9	(23)
Environmental liabilities, Ending balance	204	207
Less: current portion	27	22
Environmental liabilities non current portion	177	185
PCB [Member]
Environmental Liabilities [Line Items]
Environmental liabilities, Beginning balance	148	172
Interest accretion	7	8
Expenditures	(11)	(8)
Revaluation adjustment	(1)	(24)
Environmental liabilities, Ending balance	143	148
Less: current portion	18	12
Environmental liabilities non current portion	125	136
Land Assessment and Remediation [Member]
Environmental Liabilities [Line Items]
Environmental liabilities, Beginning balance	59	67
Interest accretion	1	2
Expenditures	(9)	(11)
Revaluation adjustment	10	1
Environmental liabilities, Ending balance	61	59
Less: current portion	9	10
Environmental liabilities non current portion	CAD 52	CAD 49

Environmental Liabilities - Rec

Environmental Liabilities - Reconciliation between Undiscounted Basis of Environmental Liabilities and Amount Recognized on Consolidated Balance Sheets (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Environmental Liabilities [Line Items]
Undiscounted environmental liabilities	CAD 224	CAD 229
Less: discounting accumulated liabilities to present value	20	22
Discounted environmental liabilities	204	207
PCB [Member]
Environmental Liabilities [Line Items]
Undiscounted environmental liabilities	158	168
Less: discounting accumulated liabilities to present value	15	20
Discounted environmental liabilities	143	148
Land Assessment and Remediation [Member]
Environmental Liabilities [Line Items]
Undiscounted environmental liabilities	66	61
Less: discounting accumulated liabilities to present value	5	2
Discounted environmental liabilities	CAD 61	CAD 59

Environmental Liabilities - 126

Environmental Liabilities - Schedule of Estimated Future Environmental Expenditures (Detail) CAD in Millions	Dec. 31, 2016CAD
Environmental Expense and Liabilities [Abstract]
2,017	CAD 27
2,018	26
2,019	25
2,020	29
2,021	36
Thereafter	81
Total	CAD 224

Environmental Liabilities - Add

Environmental Liabilities - Additional Information (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Environmental Liabilities [Line Items]
Long-term inflation rate assumption of current costs	2.00%
Reduction of environmental liability due to revaluation adjustment	CAD 9	CAD (23)
Undiscounted environmental liabilities	CAD 224	229
Minimum [Member]
Environmental Liabilities [Line Items]
Future environmental expenditure discount rate	2.00%
Maximum [Member]
Environmental Liabilities [Line Items]
Future environmental expenditure discount rate	6.30%
Land Assessment and Remediation [Member]
Environmental Liabilities [Line Items]
Reduction of environmental liability due to revaluation adjustment	CAD 10	1
Undiscounted environmental liabilities	66	61
PCB [Member]
Environmental Liabilities [Line Items]
Reduction of environmental liability due to revaluation adjustment	(1)	(24)
Undiscounted environmental liabilities	CAD 158	CAD 168

Asset Retirement Obligations -

Asset Retirement Obligations - Additional Information (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligations [Line Items]
Long-term inflation assumption of current costs	2.00%
Asset retirement obligations recorded	CAD 9	CAD 9
Minimum [Member]
Asset Retirement Obligations [Line Items]
Discounted future expenditures	3.00%
Maximum [Member]
Asset Retirement Obligations [Line Items]
Discounted future expenditures	5.00%

Share Capital - Additional Info

Share Capital - Additional Information (Detail) - CAD CAD / shares in Units, CAD in Millions	Oct. 30, 2015	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common shares, issued and outstanding		142,239	142,239
Return of stated capital, total amount paid		CAD 609	CAD 0
Preferred shares issued		0
Preferred shares outstanding		0
Cumulative Preferred Shares [Member]
Class of Stock [Line Items]
Preferred shares, issued	12,920,000
Preferred shares, outstanding	12,920,000
Cumulative preferred shares, percentage	5.50%
Cumulative preferred shares value per share	CAD 25
Cumulative preferred shares, total value	CAD 323
Preferred shares dividend, amount	CAD 18
Preferred shares dividend, per share	CAD 1.375

Share Capital - Number of Commo

Share Capital - Number of Common Shares (Detail) - CAD CAD in Millions	Nov. 05, 2015	Dec. 31, 2015	Nov. 04, 2015	Oct. 31, 2015
Common Equity [Line Items]
Common value issued		CAD 2,923
Common shares issued		42,239	39,598	2,640
Common shares issued, reverse split	1
Common Stock Issued on October 31, 2015 [Member]
Common Equity [Line Items]
Common value issued				CAD 323
Common shares issued				2,640
Common Stock Issued on November 4, 2015 [Member]
Common Equity [Line Items]
Common value issued			CAD 2,600
Common shares issued			39,598
Common Stock Issued on November 5, 2015 [Member]
Common Equity [Line Items]
Common shares issued	1

Share Capital - Number of Co131

Share Capital - Number of Common Shares (Parenthetical) (Detail) - CAD CAD in Millions	Nov. 04, 2015	Dec. 31, 2015	Oct. 31, 2015
Common Equity [Line Items]
Common shares issued	39,598	42,239	2,640
Proceeds from issuance of common stock	CAD 2,600	CAD 2,600
Cumulative Preferred Shares [Member]
Common Equity [Line Items]
Preferred share value purchased and cancelled			CAD 323
Preferred share purchased and cancelled, shares			12,920,000

Dividends - Additional Informat

Dividends - Additional Information (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Dividends [Abstract]
Dividends on preferred shares		CAD 13
Common share dividends	CAD 2	CAD 875

Earnings Per Share - Additional

Earnings Per Share - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Weighted average number of shares outstanding	142,239	107,116
Dilutive securities	0	0

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - CAD	Dec. 15, 2015	Dec. 31, 2016	Dec. 31, 2015	Sep. 01, 2015	Apr. 01, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-Average price, Granted (non-vested)		CAD 20.50
Regulatory assets		CAD 3,145,000,000	CAD 3,015,000,000
Deferred Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expenses		2,000,000
Liability related to outstanding DSUs		2,000,000
Employee Share Ownership Plan (ESOP) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Highest percentage of participant's salary towards purchasing common shares	6.00%
Lowest percentage of participant's salary towards purchasing common shares	1.00%
Employer matching contribution, percent of match	50.00%
Maximum amount contributed by employer		25,000
Contribution under the plan		CAD 2,000,000	0
Hydro One Limited [Member] | Deferred Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Closing share price of common shares		CAD 23.58
Maximum [Member] | Deferred Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expenses			1,000,000
Liability related to outstanding DSUs			CAD 1,000,000
PWU Share Grant Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Requisite service period, Beginning date		Jul. 3, 2015
Highest percentage of participant's salary towards purchasing common shares					2.70%
Share granted		3,952,212
PWU Share Grant Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Requisite service period		35 years
Aggregate number of common shares issuable under the plan		3,981,763
Society Share Grant Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Requisite service period, Beginning date		Sep. 1, 2015
Highest percentage of participant's salary towards purchasing common shares				2.00%
Share granted		1,367,158
Society Share Grant Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Requisite service period		35 years
Aggregate number of common shares issuable under the plan		1,434,686
Share Grant Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share granted			5,319,370
Weighted-Average price, Granted (non-vested)			CAD 20.50
Fair value of shares granted		CAD 0	CAD 111,000,000
Compensation expenses		21,000,000	10,000,000
Long Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expenses		3,000,000	0
Grant date total fair value of awards		CAD 12,000,000	CAD 0
Long Term Incentive Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of common shares issuable under the plan		11,900,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Share Grant Activity (Detail) - CAD / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-Average price, Granted (non-vested)	CAD 20.50
Share Grant Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share grants outstanding, Beginning	5,319,370
Share Granted (non-vested)		5,319,370
Shares Forfeited	(79,692)
Share grants outstanding, Ending	5,239,678	5,319,370
Weighted-Average price, Outstanding, Beginning	CAD 20.50
Weighted-Average price, Granted (non-vested)		CAD 20.50
Weighted-Average price, Forfeited	20.50
Weighted-Average price, Outstanding, Ending	CAD 20.50	CAD 20.50

Stock-Based Compensation - S136

Stock-Based Compensation - Summary of Number of DSUs (Detail) - Deferred Share Units [Member] - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
DSUs outstanding - January 1	20,525
DSUs granted	78,558	20,525
DSUs outstanding - December 31	99,083	20,525

Stock-Based Compensation - S137

Stock-Based Compensation - Summary of Number of PSUs and RSUs (Detail)	12 Months Ended
	Dec. 31, 2016shares
Number of PSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units granted	233,710
Units forfeited	(4,820)
Units outstanding - December 31, 2016	228,890
Number of RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units granted	257,260
Units forfeited	(4,820)
Units outstanding - December 31, 2016	252,440

Noncontrolling Interest - Addit

Noncontrolling Interest - Additional Information (Detail) - B2M Limited Partnership [Member] - CAD CAD in Millions	Dec. 17, 2014	Dec. 16, 2014
Noncontrolling Interest [Line Items]
Business combination assets transferred		CAD 526
Debt [Member]
Noncontrolling Interest [Line Items]
Business combination assets transferred		CAD 316
Business combination percentage transferred		60.00%
Equity [Member]
Noncontrolling Interest [Line Items]
Business combination assets transferred		CAD 210
Business combination percentage transferred		40.00%
Saugeen Ojibway Nation (SON) [Member]
Noncontrolling Interest [Line Items]
Percentage of common shares acquired	34.20%
Business acquisition, consideration paid	CAD 72
Class A Units [Member] | Saugeen Ojibway Nation (SON) [Member]
Noncontrolling Interest [Line Items]
Capital units in initial investment	50
Class B Units [Member] | Saugeen Ojibway Nation (SON) [Member]
Noncontrolling Interest [Line Items]
Capital units in initial investment	CAD 22

Noncontrolling Interest - Sched

Noncontrolling Interest - Schedule of Movements in Noncontrolling Interest (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Noncontrolling Interest [Abstract]
Noncontrolling interest - Beginning balance, Temporary Equity	CAD 23	CAD 21
Distributions to noncontrolling interest, Temporary Equity	(3)	(1)
Net income attributable to noncontrolling interest, Temporary Equity	2	3
Noncontrolling interest - Ending balance, Temporary Equity	22	23
Noncontrolling interest - Beginning balance, Equity	52	49
Distributions to noncontrolling interest, Equity	(6)	(4)
Net income (loss) attributable to noncontrolling interest, Equity	4	7
Noncontrolling interest - Ending balance, Equity	50	52
Noncontrolling interest - Beginning balance	75	70
Distributions to noncontrolling interest	(9)	(5)
Net income attributable to noncontrolling interest	6	10
Noncontrolling interest - Ending balance	CAD 72	CAD 75

Related Party Transactions - Su

Related Party Transactions - Summary of Related Party Transactions (Detail) - CAD CAD in Millions	Nov. 04, 2015	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Payments in lieu of corporate income taxes		CAD 30	CAD 2,928
Power purchased		3,427	3,450
Revenues for transmission services		1,587	1,536
Distribution revenues related to the supply of electricity to remote northern communities		4,915	4,949
Common shares issued	CAD 2,600		2,600
Province Of Ontario [Member]
Related Party Transaction [Line Items]
Dividends paid			888
IESO [Member]
Related Party Transaction [Line Items]
Power purchased		2,096	2,318
Revenues for transmission services		1,549	1,548
Distribution revenues related to rural rate protection		125	127
Distribution revenues related to the supply of electricity to remote northern communities		32	32
Funding received related to Conservation and Demand Management programs		63	70
OPG [Member]
Related Party Transaction [Line Items]
Power purchased		6	11
Costs expensed related to the purchase of services		1	1
OPG [Member] | Transmission [Member]
Related Party Transaction [Line Items]
Revenues related to provision of construction and equipment maintenance services		4	7
OEFC [Member]
Related Party Transaction [Line Items]
Payments in lieu of corporate income taxes			2,933
Power purchased		1	6
Indemnification fee paid (terminated effective October 31, 2015)			8
Hydro One Brampton [Member]
Related Party Transaction [Line Items]
Costs expensed related to the purchase of services		3	1
Hydro One Telecom [Member]
Related Party Transaction [Line Items]
Revenues for services provided		3
Hydro One Telecom [Member] | Operating Expense [Member]
Related Party Transaction [Line Items]
Services received		24	4
Hydro One Telecom [Member] | Capitalized Costs [Member]
Related Party Transaction [Line Items]
Services received		12	2
Hydro One Limited [Member]
Related Party Transaction [Line Items]
Common shares issued	CAD 2,600		2,600
Return of stated capital		609
Dividends paid		2
Stock-based compensation costs		24	10
IPO costs subsequently reimbursed by Hydro One Limited			7
OEB [Member]
Related Party Transaction [Line Items]
OEB fees		CAD 11	CAD 12

Related Party Transactions -141

Related Party Transactions - Summary of Related Party Transactions (Parenthetical) (Detail) - CAD CAD in Millions	Nov. 04, 2015	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2015
Related Party Transaction [Line Items]
Payment made in lieu of corporate income taxes		CAD 30	CAD 2,928
Common shares issued	39,598		42,239	2,640
Proceeds from issuance of common stock	CAD 2,600		CAD 2,600
OEFC [Member]
Related Party Transaction [Line Items]
Payment made in lieu of corporate income taxes			2,933
Departure tax included in income tax paid			2,600
Hydro One Limited [Member]
Related Party Transaction [Line Items]
Common shares issued	39,598
Proceeds from issuance of common stock	CAD 2,600		2,600
IPO related cost			CAD 7

Related Party Transactions - Sc

Related Party Transactions - Schedule of Amounts Due to and from Related Parties (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Related Party Transactions [Abstract]
Due from related parties	CAD 224	CAD 184
Due to related parties	CAD (279)	CAD (142)

Related Party Transactions -143

Related Party Transactions - Schedule of Amounts Due to and from Related Parties (Parenthetical) (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction Due From To Related Party [Line Items]
Due to related parties, amounts owing on power purchases	CAD 253	CAD 132
IESO [Member]
Related Party Transaction Due From To Related Party [Line Items]
Due to related parties, amounts owing on power purchases	CAD 143	CAD 134

Consolidated Statements of C144

Consolidated Statements of Cash Flows - Schedule of Consolidated Statement of Cash Flows (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Accounts receivable	CAD (59)	CAD 249
Due from related parties	(40)	40
Materials and supplies	2	2
Prepaid expenses and other assets	(17)	12
Accounts payable	18	(26)
Accrued liabilities	52	(27)
Due to related parties	113	(95)
Accrued interest	9	(4)
Long-term accounts payable and other liabilities	6
Post-retirement and post-employment benefit liability	84	41
Changes in non-cash balances related to operations, Total	168	192
Capital Expenditures
Capital investments in property, plant and equipment	(1,624)	(1,622)
Capitalized depreciation and net change in accruals included in capital investments in property, plant and equipment	30	28
Capital expenditures - property, plant and equipment	(1,594)	(1,594)
Capital investments in intangible assets	(67)	(40)
Net change in accruals included in capital investments in intangible assets	6	3
Capital expenditures - intangible assets	(61)	(37)
Supplementary Information
Net interest paid	418	416
Income taxes / PILs paid	CAD 30	CAD 2,928

Consolidated Statements of C145

Consolidated Statements of Cash Flows - Additional Information (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Capital contribution received	CAD 21	CAD 57

Contingencies - Additional Info

Contingencies - Additional Information (Detail) - CAD	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Payments made	CAD 1,000,000	CAD 1,000,000
Maximum [Member]
Loss Contingencies [Line Items]
Plaintiff sought value	CAD 125,000,000

Commitments - Summary of Commit

Commitments - Summary of Commitments Under Leases, Outsourcing and Other Agreements Due (Detail) CAD in Millions	Dec. 31, 2016CAD
Commitments and Contingencies Disclosure [Abstract]
Outsourcing agreements, 2017	CAD 165
Outsourcing agreements, 2018	102
Outsourcing agreements, 2019	94
Outsourcing agreements, 2020	2
Outsourcing agreements, 2021	2
Outsourcing agreements, Thereafter	9
Long-term software/meter agreement, 2017	17
Long-term software/meter agreement, 2018	17
Long-term software/meter agreement, 2019	16
Long-term software/meter agreement, 2020	17
Long-term software/meter agreement, 2021	1
Long-term software/meter agreement, Thereafter	5
Operating lease commitments, 2017	9
Operating lease commitments, 2018	9
Operating lease commitments, 2019	4
Operating lease commitments, 2020	8
Operating lease commitments, 2021	2
Operating lease commitments, Thereafter	CAD 2

Commitments - Additional Inform

Commitments - Additional Information (Detail) CAD in Millions	12 Months Ended
	Dec. 31, 2016CADDistributor	Dec. 31, 2015CAD
Commitments [Line Items]
Lease payments	CAD 10	CAD 6
Number of distributor | Distributor	1
Letters of credit provided	CAD 24	15
Letters of credit outstanding relating to retirement compensation arrangements	150	139
IESO [Member]
Commitments [Line Items]
Letters of credit provided	CAD 17	15
Inergi LP [Member] | Customer Service Operations Outstanding Services [Member]
Commitments [Line Items]
Agreement expiry date	2018-02
Inergi LP [Member] | Services Including Settlements, Source To Pay, Information Technology, Pay Operation, Finance And Accounting [Member]
Commitments [Line Items]
Agreement expiry date	2019-12
Brookfield Global Integrated Solutions [Member] | Services Including Facilities Management And Execution Of Certain Capital Projects [Member]
Commitments [Line Items]
Agreement expiry date	2024-12
Operating Leases [Member] | Minimum [Member]
Commitments [Line Items]
Typical terms of irrevocable operating lease	3 years
Operating lease renewal options	3 years
Operating Leases [Member] | Maximum [Member]
Commitments [Line Items]
Typical terms of irrevocable operating lease	5 years
Operating lease renewal options	5 years
Long-Term Software/Meter Agreement [Member] | Trilliant Agreement [Member]
Commitments [Line Items]
Agreement expiry date	2025-12
Agreement renewal term	5 years
Subsidiaries [Member]
Commitments [Line Items]
Parental guarantees	CAD 329	329
Distributor [Member]
Commitments [Line Items]
Parental guarantees	CAD 1	CAD 1

Segmented Reporting - Additiona

Segmented Reporting - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016CompaniesSegment
Segment Reporting Disclosure [Line Items]
Number of reportable segments | Segment	3
Minimum [Member]
Segment Reporting Disclosure [Line Items]
Number of local distribution companies | Companies	70

Segmented Reporting - Summary o

Segmented Reporting - Summary of Segment Information (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	CAD 6,502	CAD 6,529
Purchased power	3,427	3,450
Operation, maintenance and administration	1,043	1,130
Depreciation and amortization	769	757
Income (loss) before financing charges and income taxes	1,263	1,192
Capital investments	1,691	1,662
Total assets	25,310	24,169
Transmission [Member]
Segment Reporting Information [Line Items]
Revenues	1,587	1,536
Operation, maintenance and administration	410	415
Depreciation and amortization	390	374
Income (loss) before financing charges and income taxes	787	747
Capital investments	988	943
Total assets	13,083	12,045
Distribution [Member]
Segment Reporting Information [Line Items]
Revenues	4,915	4,949
Purchased power	3,427	3,450
Operation, maintenance and administration	613	633
Depreciation and amortization	379	380
Income (loss) before financing charges and income taxes	496	486
Capital investments	703	711
Total assets	9,393	9,200
Other [Member]
Segment Reporting Information [Line Items]
Revenues		44
Operation, maintenance and administration	20	82
Depreciation and amortization		3
Income (loss) before financing charges and income taxes	(20)	(41)
Capital investments		8
Total assets	CAD 2,834	CAD 2,924

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) CAD in Millions	Feb. 09, 2017CAD
Subsequent Event [Member]
Subsequent Event [Line Items]
Return of stated capital, total amount approved	CAD 147