Common stock buyback program stock option and award plan: (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
	Oct. 01, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Remaining authorized amount for common stock repurchases		$ 34.6				$ 34.6
Repurchase of common stock (in shares)	53,516	4,567			0	4,567	0
Cost of shares of common stock	$ 3.1	$ 0.3				$ 0.3
Performance conditions
Stock Repurchase Program, Number of Shares Authorized to be Repurchased				94,050
Executive employees and directors
Shares issue (in shares)			135,280	319,750
Shares issued, Value			$ 10.6	$ 23.7
CEO | Performance conditions
Stock Repurchase Program, Number of Shares Authorized to be Repurchased				35,000