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Shareholders' capital - Additional Information (Detail) CAD / shares in Units, CAD in Thousands |  |  | 2 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Nov. 10, 2017 CAD CAD / shares shares | Dec. 29, 2014 CAD CAD / shares shares | Mar. 02, 2018 shares | Jun. 30, 2016 shares | Dec. 31, 2017 CAD Right vote CAD / shares shares | Dec. 31, 2016 CAD CAD / shares shares
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Number of entitled votes per common share | vote |  |  |  |  | 1 |
Number of voting rights per share | Right |  |  |  |  | 1 |
Discount rate on share purchases |  |  |  |  | 50.00% |
Common shares issued (in shares) |  |  |  |  | 43,470,000 | 12,938,457
Discount rate on share purchases under dividend reinvestment plan |  |  |  |  | 5.00% |
Carrying amount | CAD |  |  |  |  | CAD 213,805 | CAD 213,805
Preferred stock redemption price per share (in CAD per share) | CAD / shares |  |  |  |  | CAD 25 |
Total share-based compensation expense | CAD |  |  |  |  | CAD 10,804 | 5,675
Death Benefits |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Stock options exercised period |  |  |  |  | 1 year |
Resignation or Termination |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Stock options exercised period |  |  |  |  | 30 days |
Maximum | Retirement |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Stock options exercised period |  |  |  |  | 90 days |
Minimum |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Percentage of outstanding stock to be purchased to acquire discount (or more) |  |  |  |  | 20.00% |
Subsequent Event |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Additional shares issued under dividend reinvestment plan (in shares) |  |  | 1,063,572 |  |  |
Employee Stock Option |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Total share-based compensation expense | CAD |  |  |  |  | CAD 3,990 | 3,006
Unrecognized compensation costs, non-vested options | CAD |  |  |  |  | CAD 2,796 |
Unrecognized compensation costs, non-vested options, period of recognition |  |  |  |  | 1 year 7 months 10 days |
Stock Option Plans | Maximum |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Percentage of shares reserves under plan (must not exceed) |  |  |  |  | 8.00% |
Performance Share Units |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Maximum aggregate number of shares reserved for future issuance (in shares) |  |  |  |  | 7,000,000 |
Performance share units granting period |  |  |  |  | 3 years |
Percentage of shares issued on number of PSU grants, minimum |  |  |  |  | 2.00% |
Percentage of shares issued on number of PSU grants, maximum |  |  |  |  | 237.00% |
Employee share purchase |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Total share-based compensation expense | CAD |  |  |  |  | CAD 568 | CAD 238
Vesting period of matching contribution shares |  |  |  |  | 1 year |
Maximum aggregate number of shares reserved for future issuance (in shares) |  |  |  |  | 2,000,000 |
Common stock, shares issued (in shares) |  |  |  |  | 283,523 | 144,264
Deferred Share Units |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Maximum aggregate number of shares reserved for future issuance (in shares) |  |  |  |  | 1,000,000 |
Shares issued during period (in shares) |  |  |  |  | 293,906 | 224,663.24
Performance share units |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Total share-based compensation expense | CAD |  |  |  |  | CAD 5,475 | CAD 1,748
Unrecognized compensation costs, non-vested awards | CAD |  |  |  |  | CAD 8,471 |
Unrecognized compensation costs, non-vested options, period of recognition |  |  |  |  | 1 year 10 months 2 days |
Series A Preferred Stock |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Preferred stock issued (in shares) |  |  |  |  | 4,800,000 | 4,800,000
Shares issued, price per share (CAD per share) | CAD / shares |  |  |  |  | CAD 25 | CAD 25
Carrying amount | CAD |  |  |  |  | CAD 116,546 | CAD 116,546
Fixed cumulative annual dividend per share (CAD per share) | CAD / shares |  |  |  |  | CAD 1.125 |
Subsequent yield period |  |  |  |  | 5 years |
Preferred stock, Subsequent redemption period |  |  |  |  | 5 years |
Series C Preferred Stock |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Preferred stock redemption price per share (in CAD per share) | CAD / shares |  |  |  |  | CAD 53,400 |
Redeemable preferred stock issued (in shares) |  |  |  |  | 100 |
Series D Preferred Stock |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Preferred stock issued (in shares) |  |  |  |  | 4,000,000 | 4,000,000
Shares issued, price per share (CAD per share) | CAD / shares |  |  |  |  | CAD 25 | CAD 25
Carrying amount | CAD |  |  |  |  | CAD 97,259 | CAD 97,259
Fixed cumulative annual dividend per share (CAD per share) | CAD / shares |  |  |  |  | CAD 1.25 |
Subsequent yield period |  |  |  |  | 5 years |
Preferred stock, Subsequent redemption period |  |  |  |  | 5 years |
Government of Canada Bond Yield | Series A Preferred Stock |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Dividend variable interest rate |  |  |  |  | 2.94% |
Government of Canada Bond Yield | Series D Preferred Stock |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Dividend variable interest rate |  |  |  |  | 3.28% |
Government of Canada Treasury Bill Yield | Series B Preferred Stock |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Dividend variable interest rate |  |  |  |  | 2.94% |
Government of Canada Treasury Bill Yield | Series E Preferred Stock |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Dividend variable interest rate |  |  |  |  | 3.28% |
Emera | Subscription receipts | Park Water Company |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Common shares issued (in shares) |  | 3,316,583 |  |  |  |
Issuance of convertible debentures, net of costs | CAD |  | CAD 33,000 |  |  |  |
Additional shares issued under dividend reinvestment plan (in shares) |  |  |  | 252,011 |  |
Shares issued, price per share (CAD per share) | CAD / shares |  | CAD 9.95 |  |  |  |
Emera | Odell Wind Facility | Subscription receipts |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Common shares issued (in shares) |  | 8,708,170 |  |  |  |
Issuance of convertible debentures, net of costs | CAD |  | CAD 77,503 |  |  |  |
Additional shares issued under dividend reinvestment plan (in shares) |  |  |  | 661,693 |  |
Shares issued, price per share (CAD per share) | CAD / shares |  | CAD 8.90 |  |  |  |
Range One | Employee share purchase |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Employer matching contribution, percent |  |  |  |  | 20.00% |
Range Two | Employee share purchase |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Employer matching contribution, percent |  |  |  |  | 10.00% |
Range Three | Employee share purchase |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Employer matching contribution, percent |  |  |  |  | 15.00% |
Public Stock Offering [Member] |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Sale of stock, price per share (CAD per share) | CAD / shares | CAD 13.25 |  |  |  |  |
Payments of Stock Issuance Costs | CAD | CAD 24,342 |  |  |  |  |
Public Stock Offering [Member] | Common shares |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Number of shares issued pursuant to public offering | 43,470,000 |  |  |  |  |
Cash proceeds from issuance of shares | CAD | CAD 576,000 |  |  |  |  |
Payments Of Stock Issuance Costs, Net of Tax | CAD | CAD 17,895 |  |  |  |  |

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