Description of the business and summary of significant accounting policies: Basic and diluted net income per common share (Details) - shares	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Diluted weighted average shares
Weighted-average common shares - basic (in shares)	44,844,469	44,767,163	44,717,372	44,649,645	44,577,826	44,574,583	44,491,899	44,402,640	44,855,263	44,641,805	44,888,723
Weighted average common shares-diluted	44,844,469	45,118,607	44,988,655	44,917,014	44,803,782	44,816,860	44,757,494	44,571,937	45,184,203	44,873,030	45,159,489
Stock options
Diluted weighted average shares
Dilutive effect (options or restricted stock)									31,534	31,760	40,196
Anti-dilutive effects
Anti-dilutive (options or restricted stock)									47,513	90,959	119,872
Restricted stock
Diluted weighted average shares
Dilutive effect (options or restricted stock)									297,406	199,465	230,570
Anti-dilutive effects
Unvested shares of restricted common stock	1,112,151				908,761				1,112,151	908,761	870,751
Anti-dilutive (options or restricted stock)									201	79,450	362,241