Commitments and contingencies: (Details 2) (USD $)	0 Months Ended			3 Months Ended
	Apr. 18, 2013	Mar. 15, 2013	Feb. 20, 2013	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Interest and penalties related to uncertain tax positions
Liability for uncertain tax positions, including accrued interest and penalties				$ 1,700,000		$ 1,700,000
Interest and penalties related to uncertain tax positions				17,000	76,000
Expected changes in the liability for uncertain tax positions
Expected decrease in liability for uncertain tax positions				400,000
Common stock buyback program
Authorized amount of common stock repurchases under the Buyback Program				50,000,000
Remaining authorized amount of common stock repurchases under the Buyback Program				45,800,000
Repurchase under the common stock buyback program				0	0
Dividends on common stock
Value of dividend declared, per common share (in dollars per share)	$ 0.13		$ 0.12	$ 0.12
Value of dividend paid		5,500,000
Estimated value of dividend to be paid on June 18, 2013	$ 6,100,000