Share Capital - Changes in Number of Shares of Class A Stock Outstanding (Details) - Class A Stock - shares	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Class A Stock outstanding, beginning of period	12,923,517	13,141,103	12,941,809	13,139,203
Issued pursuant to shared-based compensation plans	0	15,250	61,091	17,150
Repurchased and canceled pursuant to the stock buy-back	167,209	0	246,592	0
Class A Stock outstanding, end of period	12,756,308	13,156,353	12,756,308	13,156,353

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Number of shares issued under share-based payment arrangement.

+ References

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- Definition

Number of shares of common and preferred stock retired from treasury during the period.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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