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Shareholders' capital - Additional Information (Detail) $ / shares in Units, $ / shares in Units, $ in Thousands, $ in Thousands |  |  |  | 12 Months Ended |  |  |  |  |
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Dec. 20, 2018 CAD ($) shares | Dec. 20, 2018 USD ($) $ / shares shares | Apr. 24, 2018 CAD ($) shares | Apr. 24, 2018 USD ($) $ / shares shares | Nov. 10, 2017 CAD ($) shares | Nov. 10, 2017 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) Right vote shares | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2023 $ / shares | Dec. 31, 2018 CAD ($) $ / shares shares | Dec. 31, 2018 USD ($) shares | Dec. 20, 2018 $ / shares | Apr. 24, 2018 $ / shares | Nov. 10, 2017 $ / 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) |  |  |  |  |  |  | 50,041,624 | 43,470,000 |  |  |  |  |  |
Discount rate on share purchases under dividend reinvestment plan |  |  |  |  |  |  | 5.00% |  |  |  |  |  |  |
Dividend reinvestment plan shares issued (in shares) |  |  |  |  |  |  | 1,606,001 |  |  |  |  |  |  |
Carrying amount C$ |  |  |  |  |  |  |  | $ 184,299 |  |  | $ 184,299 |  |  |
Preferred stock redemption price per share (in CAD per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 25 |  |  |  |
Total share-based compensation expense | $ |  |  |  |  |  |  | $ 9,458 | 8,361 |  |  |  |  |  |
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% |  |  |  |  |  |  |
Employee Stock Option |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total share-based compensation expense | $ |  |  |  |  |  |  | $ 2,054 | 3,070 |  |  |  |  |  |
Unrecognized compensation costs, non-vested options | $ |  |  |  |  |  |  |  |  |  |  | $ 1,221 |  |  |
Unrecognized compensation costs, non-vested options, period of recognition |  |  |  |  |  |  | 1 year 7 months 21 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 | 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 | $ |  |  |  |  |  |  | $ 312 | $ 436 |  |  |  |  |  |
Vesting period of matching contribution shares |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |
Maximum aggregate number of shares reserved for future issuance (in shares) |  |  |  |  |  |  |  |  |  | 2,000,000 | 2,000,000 |  |  |
Common stock, shares issued (in shares) |  |  |  |  |  |  |  | 283,523 |  | 252,698 | 252,698 |  |  |
Deferred Share Units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum aggregate number of shares reserved for future issuance (in shares) |  |  |  |  |  |  |  |  |  | 1,000,000 | 1,000,000 |  |  |
Shares issued during period (in shares) |  |  |  |  |  |  | 380,656 | 293,906.32 |  |  |  |  |  |
Performance and restricted share units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total share-based compensation expense | $ |  |  |  |  |  |  | $ 6,378 | $ 4,262 |  |  |  |  |  |
Unrecognized compensation costs, non-vested awards | $ |  |  |  |  |  |  |  |  |  |  | $ 8,243 |  |  |
Unrecognized compensation costs, non-vested options, period of recognition |  |  |  |  |  |  | 1 year 7 months 5 days |  |  |  |  |  |  |
Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock issued (in shares) |  |  |  |  |  |  |  | 4,800,000 |  | 4,800,000 | 4,800,000 |  |  |
Shares issued, price per share (CAD per share) | (per share) |  |  |  |  |  |  |  | $ 25 |  | $ 25 |  |  |  |
Carrying amount C$ |  |  |  |  |  |  |  | $ 100,463 |  | $ 116,546 | $ 100,463 |  |  |
Fixed cumulative annual dividend per share (CAD per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 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) | $ / shares |  |  |  |  |  |  |  |  |  | $ 53,400 |  |  |  |
Redeemable preferred stock issued (in shares) |  |  |  |  |  |  |  |  |  | 100 | 100 |  |  |
Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock issued (in shares) |  |  |  |  |  |  |  | 4,000,000 |  | 4,000,000 | 4,000,000 |  |  |
Shares issued, price per share (CAD per share) | (per share) |  |  |  |  |  |  |  | $ 25 |  | $ 25 |  |  |  |
Carrying amount C$ |  |  |  |  |  |  |  | $ 83,836 |  | $ 97,259 | $ 83,836 |  |  |
Fixed cumulative annual dividend per share (CAD per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 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% |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sale of stock, price per share (CAD per share) | (per share) |  | $ 10.09 |  | $ 9.23 |  | $ 10.45 |  |  |  |  |  | $ 13.76 | $ 11.85 | $ 13.25
Payments of Stock Issuance Costs | $ 492 | $ 366 | $ 765 | $ 590 | $ 24,342 | $ 19,193 |  |  |  |  |  |  |  |
Public Stock Offering | Common shares |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued pursuant to public offering | 12,536,350 | 12,536,350 | 37,505,274 | 37,505,274 | 43,470,000 | 43,470,000 |  |  |  |  |  |  |  |
Cash proceeds from issuance of shares | $ 172,500 | $ 126,485 | $ 444,437 | $ 346,458 | $ 576,000 | $ 454,158 |  |  |  |  |  |  |  |
Payments Of Stock Issuance Costs, Net of Tax |  |  |  |  | $ 17,895 | $ 14,109 |  |  |  |  |  |  |  |
Scenario, Forecast | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed cumulative annual dividend per share (CAD per share) | $ / shares |  |  |  |  |  |  |  |  | $ 1.2905 |  |  |  |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested options awarded to employees as compensation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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