Employee benefit plans: (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Employee benefit plans:
Maximum percentage of eligible annual compensation that the employee can contribute subject to IRS limitations	60.00%
Percentage of employee's contribution percent matched	75.00%
Employer matching contribution, as a percent of employee's eligible compensation	6.00%
Amount of contributions to the matching feature of the 401(k) plan	$ 1,161,000	$ 1,016,000	$ 925,000
Contribution to employer retirement contribution feature (as a percent)	8.00%
Amount of contributions to the employer retirement contribution feature of the 401(k) plan	$ 2,289,000	$ 2,029,000	$ 1,844,000