Capital Stock (Summary of Common Stock Activity) (Details) - shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common shares outstanding at beginning of period (in shares)	135,577,878	135,173,948	134,275,609
Shares repurchased (in shares)	(1,240,328)	(106,388)	(92,167)
Shares issued upon exercise of employee stock options (in shares)	0	70,000	0
Common shares outstanding at end of period (in shares)	134,801,030	135,577,878	135,173,948
Restricted stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Restricted stock issued to employees, net of forfeitures (in shares)	439,700	416,695	958,672
Share-based Payment Arrangement \| Share-based Payment Arrangement, Nonemployee
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Restricted stock issued to employees, net of forfeitures (in shares)	23,780	23,623	31,834

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

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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

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

+ Details

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- Definition

Number of share options (or share units) exercised during the current period.

+ References

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- Definition

Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

+ References

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