Stockholders' equity: Common stock buybacks (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common stock buyback program stock option and award plan:
Authorized amount for common stock repurchases	$ 50.0
Remaining authorized amount for common stock repurchases	$ 30.4
Repurchase of common stock (in shares)	79,056	0	147,995
Cost of shares of common stock	$ 4.5		$ 6.6

Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate 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.

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