Income taxes: Effective Income Tax Rate Reconciliation (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) subsidiary	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Income taxes:
Number of potential subsidiaries filing income tax returns | subsidiary	1
Reconciliation of the Federal statutory income taxes to the amounts reported in the financial statements
Federal income tax expense at statutory rates	$ (10,892)	$ (8,492)	$ (4,450)
State income tax, net of federal benefit	(2,244)	(1,080)	(1,710)
Impact of foreign operations	74	138	(179)
Non-deductible expenses	(1,350)	(590)	(1,253)
Changes in tax reserves		175	128
Federal tax rate change	(9,046)
Transition tax on foreign earnings	(2,296)
Other	239	98	(16)
Change in valuation allowance	273	420	(336)
Total income tax expense	$ (25,242)	$ (9,331)	$ (7,816)