Stock-Based Compensation - Weighted-Average Fair Value Assumptions (Details) - CAD ($)	3 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividends declared per share	$ 0.9500	$ 3.5600	$ 3.1400	$ 2.5125
Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option life (years)		4 years 9 months	5 years	5 years
Risk-free interest rate		1.28%	2.22%	2.22%
Expected stock price volatility		23.14%	25.04%	24.81%
Expected annual dividends per share		$ 3.3200	$ 2.6191	$ 2.3854
Estimated forfeiture rate		4.41%	6.05%	4.70%
Weighted average grant date fair value of options granted during the year		$ 69.00	$ 63.69	$ 55.63
Expected annual dividends per share		$ 3.8000

Company estimated forfeitures based on past experience.

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award, Fair Value Assumptions, Expected Dividends Per Share

+ References

No definition available.

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- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
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- Definition

The estimated amount of dividends to be paid to holders of the underlying shares (expected dividends) over the option's term. Dividends are taken into account because payment of dividends to shareholders reduces the fair value of the underlying shares, and option holders generally do not receive dividends.

+ References

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
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