Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified					9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2013 Minimum	Sep. 30, 2013 Maximum	Sep. 30, 2013 6.7% Senior Notes	Sep. 30, 2013 Fair Value Level 1	Dec. 31, 2012 Fair Value Level 1	Sep. 30, 2013 Fair Value Level 1 6.95% Senior Notes	Dec. 31, 2012 Fair Value Level 1 6.95% Senior Notes	Sep. 30, 2013 Fair Value Level 2 6.7% Senior Notes	Dec. 31, 2012 Fair Value Level 2 6.7% Senior Notes	Sep. 30, 2013 Fair Value U.S. Treasury Notes Level 1	Dec. 31, 2012 Fair Value U.S. Treasury Notes Level 1	Sep. 30, 2013 Book Value	Dec. 31, 2012 Book Value	Sep. 30, 2013 Book Value 6.95% Senior Notes	Dec. 31, 2012 Book Value 6.95% Senior Notes	Sep. 30, 2013 Book Value 6.7% Senior Notes	Dec. 31, 2012 Book Value 6.7% Senior Notes	Sep. 30, 2013 Book Value U.S. Treasury Notes	Dec. 31, 2012 Book Value U.S. Treasury Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 183,101	$ 378,358	$ 409,579	$ 424,155				$ 183,101	$ 378,358							$ 183,101	$ 378,358
Short-term investments	45,162	100,676												45,162	100,676							45,162	100,676
Long-term investments	40,099	50,305												40,154	50,339							40,099	50,305
Long-term debt										$ 330,509	$ 376,610	$ 512,221	$ 582,744					$ 342,000	$ 342,000	$ 532,383	$ 532,194
Long term investment maturities					Nov. 30, 2014	Dec. 15, 2014
Fair value assumption, interest rate							7.25%