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Shareholders' capital - Additional Information (Detail) $ / shares in Units, $ / shares in Units, $ in Thousands |  |  |  | 1 Months Ended | 2 Months Ended | 3 Months Ended | 12 Months Ended | 23 Months Ended |  |
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Jul. 17, 2020 USD ($) $ / shares shares | Jul. 17, 2020 CAD ($) shares | Mar. 31, 2019 shares | Mar. 30, 2019 $ / shares | Oct. 31, 2019 USD ($) $ / shares shares | Mar. 04, 2021 shares | Dec. 31, 2020 USD ($) shares | Dec. 31, 2020 USD ($) vote right $ / shares shares | Dec. 31, 2020 USD ($) $ / shares shares | Dec. 31, 2019 USD ($) shares | Dec. 31, 2019 USD ($) $ / shares | Dec. 31, 2020 USD ($) $ / shares shares | Dec. 31, 2020 CAD ($) $ / shares shares | Jul. 17, 2020 $ / 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 (percent) |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |
Discount rate on share purchases under dividend reinvestment plan (percent) |  |  |  |  |  |  |  | 5.00% |  |  |  |  |  |
Carrying amount | $ |  |  |  |  |  |  | $ 184,299,000 | $ 184,299,000 | $ 184,299,000 | $ 184,299,000 | $ 184,299,000 | $ 184,299,000 |  |
Share-based compensation expense | $ |  |  |  |  |  |  |  | 24,637,000 |  | $ 11,042,000 |  |  |  |
Unrecognized compensation costs, non-vested awards | $ |  |  |  |  |  |  | $ 12,063,000 | $ 12,063,000 | $ 12,063,000 |  |  | $ 12,063,000 |  |
Unrecognized compensation costs, non-vested options, period of recognition |  |  |  |  |  |  |  | 1 year 8 months 15 days |  |  |  |  |  |
Succession and retirement expense | $ |  |  |  |  |  |  |  | $ 2,582,000 |  |  |  |  |  |
Options granted (in shares) |  |  |  |  |  |  |  | 999,962 |  | 1,113,775 |  |  |  |
Options granted (CAD per share) | $ / shares |  |  |  |  |  |  |  |  | $ 16.78 |  | $ 14.96 |  |  |
Options exercised (in shares) |  |  |  |  |  |  |  | 2,386,275 |  | 3,882,505 |  |  |  |
Options exercised (CAD per share) | $ / shares |  |  |  |  |  |  |  |  | $ 12.52 |  | 11.23 |  |  |
Total share-based compensation expense | $ |  |  |  |  |  |  |  | $ 24,637,000 |  | $ 11,042,000 |  |  |  |
Company Match, First $5,000 Contributed by Employee |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employer matching contribution (percent) |  |  |  |  |  |  |  | 20.00% |  |  |  |  |  |
Company Match, Employee Contributions $5,001 to $10,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employer matching contribution (percent) |  |  |  |  |  |  |  | 10.00% |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of outstanding stock to be purchased to acquire discount (or more) |  |  |  |  |  |  |  | 20.00% |  |  |  |  |  |
Common shares |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options settled at cash value for payment of the exercise price and for tax withholdings (in shares) |  |  |  |  |  |  | 7,377 |  |  |  |  |  |  |
Treasury Stock, Common |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued from treasury for option exercise (in shares) |  |  |  |  |  |  |  | 6,401,000 |  |  |  |  |  |
Retirement Restricted Share Units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Awards vested (percent) |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |
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 | 7,000,000 |  |  | 7,000,000 | 7,000,000 |
Award vesting period |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |
Performance Share Units | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of shares issued on number of PSU grants (percent) |  |  |  |  |  |  |  | 2.50% |  |  |  |  |  |
Performance Share Units | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of shares issued on number of PSU grants (percent) |  |  |  |  |  |  |  | 237.00% |  |  |  |  |  |
Bonus Deferral Restricted Stock Units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued during period (in shares) |  |  |  |  |  |  |  | 135,409 |  |  |  |  |  |
Deferred Share Units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued (in shares) |  |  |  |  |  |  | 13,778,000 | 13,778,000 | 13,778,000 |  |  | 13,778,000 | 13,778,000 |
Maximum aggregate number of shares reserved for future issuance (in shares) |  |  |  |  |  |  | 1,000,000 | 1,000,000 | 1,000,000 |  |  | 1,000,000 | 1,000,000 |
Shares issued during period (in shares) |  |  |  |  |  |  |  | 544,493 |  | 460,418 |  |  |  |
Employee Stock Option |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based compensation expense | $ |  |  |  |  |  |  |  | $ 1,743,000 |  | $ 1,288,000 |  |  |  |
Percentage of shares reserved under the plan (must not exceed) |  |  |  |  |  |  |  | 8.00% |  |  |  |  |  |
Total share-based compensation expense | $ |  |  |  |  |  |  |  | $ 1,743,000 |  | $ 1,288,000 |  |  |  |
ESPP |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period of matching contribution shares |  |  |  |  |  |  |  | 1 year |  |  |  |  |  |
Maximum aggregate number of shares reserved for future issuance (in shares) |  |  |  |  |  |  | 4,000,000 | 4,000,000 | 4,000,000 |  |  | 4,000,000 | 4,000,000 |
Shares issued during period (in shares) |  |  |  |  |  |  |  | 302,727 |  | 253,538 |  |  |  |
Subsequent Event | Common shares |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend reinvestment plan shares issued (in shares) |  |  |  |  |  | 1,403,635 |  |  |  |  |  |  |  |
ATM Equity Program |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued pursuant to public offering (in shares) |  |  |  |  |  |  |  | 8,664,563 |  |  |  | 10,421,362 |  |
Cash proceeds from issuance of shares | $ |  |  |  |  |  |  |  | $ 120,634,000 |  |  |  | $ 142,668,000 |  |
Treasury atock, amount reserved for issuance under the plan | $ |  |  |  |  |  |  | $ 500,000,000 | $ 500,000,000 | $ 500,000,000 |  |  | $ 500,000,000 |  |
Sale of stock, average price per share (in USD) | $ / shares |  |  |  |  |  |  |  | $ 13.92 |  |  |  | $ 13.69 |  |
Gross proceeds from sale of stock, net of commissions | $ |  |  |  |  |  |  |  | $ 119,126,000 |  |  |  | $ 140,885,000 |  |
Sale of stock, other related costs | $ |  |  |  |  |  |  |  | $ 1,346,000 |  |  |  | $ 3,413,000 |  |
Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued, price per share (USD and CAD per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  | $ 25 |
Preferred stock issued (in shares) |  |  |  |  |  |  | 4,800,000 | 4,800,000 | 4,800,000 |  |  | 4,800,000 | 4,800,000 |
Carrying amount |  |  |  |  |  |  | $ 100,463,000 | $ 100,463,000 | $ 100,463,000 |  |  | $ 100,463,000 | $ 116,546 |
Dividend declared per preferred share (CAD per share) | (per share) |  |  |  |  |  |  |  | $ 1.2905 |  |  | 1.2905 |  |  |
Preferred dividend rate reset period |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |
Basis spread on five-year Government of Canada Bond Yield (percent) |  |  |  |  |  |  |  | 2.94% |  |  |  |  |  |
Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued, price per share (USD and CAD per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  | $ 25 |
Preferred stock issued (in shares) |  |  |  |  |  |  | 4,000,000 | 4,000,000 | 4,000,000 |  |  | 4,000,000 | 4,000,000 |
Carrying amount |  |  |  |  |  |  | $ 83,836,000 | $ 83,836,000 | $ 83,836,000 |  |  | $ 83,836,000 | $ 97,259 |
Dividend declared per preferred share (CAD per share) | (per share) |  |  |  | $ 1.25 |  |  |  | $ 1.2728 |  |  | $ 1.2671 |  |  |
Preferred dividend rate reset period |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |
Preferred dividend, initial period |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |
Basis spread on five-year Government of Canada Bond Yield (percent) |  |  |  |  |  |  |  | 3.28% |  |  |  |  |  |
Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible preferred stock, shares issued upon conversion |  |  | 0 |  |  |  |  |  |  |  |  |  |  |
Series C Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock issued (in shares) |  |  |  |  |  |  | 100 | 100 | 100 |  |  | 100 | 100 |
Public Stock Offering | Common shares |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued pursuant to public offering (in shares) | 57,465,500 | 57,465,500 |  |  | 26,252,542 |  |  |  |  |  |  |  |  |
Shares issued, price per share (USD and CAD per share) | (per share) | $ 12.60 |  |  |  | $ 13.50 |  |  |  |  |  |  |  |  | $ 17.10
Cash proceeds from issuance of shares | $ 723,926,000 | $ 982,660 |  |  | $ 354,409,000 |  |  |  |  |  |  |  |  |
Issuance costs | $ 25,268,000 | $ 34,299 |  |  | $ 14,418,000 |  |  |  |  |  |  |  |  |
Other Net Losses |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based compensation expense | $ |  |  |  |  |  |  |  | $ 12,639,000 |  |  |  |  |  |
Total share-based compensation expense | $ |  |  |  |  |  |  |  | 12,639,000 |  |  |  |  |  |
Other Net Losses | Retirement Restricted Share Units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based compensation expense | $ |  |  |  |  |  |  |  | 5,466,000 |  |  |  |  |  |
Total share-based compensation expense | $ |  |  |  |  |  |  |  | 5,466,000 |  |  |  |  |  |
Other Net Losses | Performance Share Units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based compensation expense | $ |  |  |  |  |  |  |  | 4,591,000 |  |  |  |  |  |
Total share-based compensation expense | $ |  |  |  |  |  |  |  | $ 4,591,000 |  |  |  |  |  |

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