Stockholders' equity: Common stock buybacks (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders' equity:
Authorized amount for common stock repurchases	$ 50.0
Remaining authorized amount for common stock repurchases	$ 41.5
Repurchase of common stock (in shares)	0.1	0.1	1.2
Cost of repurchase of common stock	$ 1.8	$ 4.5	$ 39.4