Common Shares (Details) - shares	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares issued	24,825,891	24,777,001
Shares outstanding	24,170,585	24,132,910
Exercisable at end of year (in shares)	932,500	1,116,750	1,039,750
Reserved For Issuance Upon Exercise Or Vesting Of Equity Awards
Number of shares reserved for future issuance	24,825,891
Stock options \| Reserved For Issuance Upon Exercise Or Vesting Of Equity Awards
Number of shares reserved for future issuance	5,600,676
Nil Coupon Perpetual Loan Notes
Number of shares reserved for future issuance	6,715

Aggregate number of common shares reserved for future issuance.

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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.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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-Publisher FASB
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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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