Employee Benefit Plans: (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended					12 Months Ended
	Dec. 31, 2012 401(k) Plan	Dec. 31, 2011 401(k) Plan	Dec. 31, 2010 401(k) Plan	Feb. 01, 1999 401(k) Plan	Jan. 31, 2013 401(k) Plan Subsequent event	Dec. 31, 2012 Deferred Compensation Plans	Dec. 31, 2011 Deferred Compensation Plans	Dec. 31, 2010 Deferred Compensation Plans
Employee Benefit Plans:
Number of common stock shares reserved for issuance				150,000
Employer match of employee contributions of first 3% of eligible compensation (as a percent)	100.00%
Number of additional common stock shares reserved for issuance					500,000
Percentage of eligible compensation, matched 100% by employer	3.00%
Employer match of employee contributions of next 2% of eligible compensation (as a percent)	50.00%
Percentage of eligible compensation, matched 50% by employer	2.00%
Employer contribution	$ 3,094	$ 3,077	$ 3,502			$ 648	$ 570	$ 586