Stockholders' equity: Common stock buyback program: (Details) - USD ($) $ in Thousands, shares in Millions	3 Months Ended							12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Common stock buyback program:
Authorized amount of common stock repurchases under the Buyback Program								$ 50,000
Remaining authorized amount of common stock repurchases under the Buyback Program								$ 47,800
Repurchase of common stock (in shares)								1.2	1.7	0.0
Stock Buyback Amount During the Period	$ 12,169	$ 19,106	$ 8,119	$ 10,555	$ 15,943	$ 17,888	$ 14,196	$ 39,400	$ 58,600