Long-term debt: (Details) (USD $)			0 Months Ended			1 Months Ended	3 Months Ended		12 Months Ended	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Aug. 19, 2013 Senior Notes	Jan. 26, 2011 Senior Notes	Mar. 31, 2014 Senior Notes	Jun. 30, 2007 Convertible senior notes	Mar. 31, 2014 Convertible senior notes	Mar. 31, 2013 Convertible senior notes	Dec. 31, 2008 Convertible senior notes	Mar. 31, 2014 Convertible senior notes June 15, 2014, 2017 and 2022	Mar. 31, 2014 Convertible senior notes After June 20, 2014
Long-term debt
Interest rate (as a percent)			8.375%	8.375%	8.375%	1.00%	1.00%	1.00%
Aggregate principal amount of debt issued			$ 65,000,000	$ 175,000,000	$ 240,000,000	$ 200,000,000	$ 92,000,000
Proceeds from issuance of long-term debt, net of issuance costs			69,900,000	170,500,000		195,100,000
Debt issuance costs			1,000,000	4,500,000
Premium price of debt instrument (as a percent)			109.00%
Senior secured notes, premium (in dollars)	5,131,000	5,423,000	5,900,000
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
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase										100.00%	100.00%
Debt and equity components for the Convertible Notes
Principal amount							91,978,000	91,978,000
Unamortized discount	(1,414,000)	(3,099,000)					(1,414,000)	(3,099,000)
Net carrying amount							90,564,000	88,879,000
Additional paid-in capital							74,933,000	74,933,000
Amount of interest expense recognized and effective interest rate
Contractual coupon interest							230,000	230,000
Amortization of discount and costs on Notes							1,689,000	1,551,000
Interest expense							$ 1,919,000	$ 1,781,000
Effective interest rate (as a percent)							8.70%	8.70%