Long-term debt: (Details) (USD $)	6 Months Ended			0 Months Ended	3 Months Ended		6 Months Ended		1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jan. 26, 2011 Senior Notes	Jun. 30, 2013 Senior Notes	Jun. 30, 2012 Senior Notes	Jun. 30, 2013 Senior Notes	Jun. 30, 2012 Senior Notes	Jun. 30, 2007 Convertible senior notes	Jun. 30, 2013 Convertible senior notes	Jun. 30, 2012 Convertible senior notes	Jun. 30, 2013 Convertible senior notes	Jun. 30, 2012 Convertible senior notes	Dec. 31, 2008 Convertible senior notes	Dec. 31, 2012 Convertible senior notes
Long-term debt
Interest rate (as a percent)				8.375%					1.00%	1.00%	1.00%	1.00%	1.00%
Aggregate principal amount of debt issued				$ 175,000,000					$ 200,000,000
Proceeds from issuance of long-term debt, net of issuance costs				170,500,000					195,100,000
Debt issuance costs				4,500,000
Interest expense related to its senior notes					3,800,000	3,800,000	7,600,000	7,600,000
Percentage of original issuance discount									2.25%
Aggregate face value of debt purchased														108,000,000
Purchase of convertible notes in cash														48,600,000
Debt and equity components for the Convertible Notes
Principal amount										91,978,000		91,978,000			91,978,000
Unamortized discount	(6,365,000)		(9,494,000)							(6,365,000)		(6,365,000)			(9,494,000)
Net carrying amount										85,613,000		85,613,000			82,484,000
Additional paid-in capital										74,933,000		74,933,000			74,933,000
Amount of interest expense recognized and effective interest rate
Remaining recognition period of unamortized discount												1 year
Contractual coupon interest										230,000	230,000	460,000	460,000
Amortization of discount and costs on Notes	3,193,000	2,959,000								1,585,000	1,456,000	3,136,000	2,880,000
Interest expense										$ 1,815,000	$ 1,686,000	$ 3,596,000	$ 3,340,000
Effective interest rate (as a percent)										8.70%	8.70%	8.70%	8.70%