Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified																					6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2014 Fair Value Level 1	Dec. 31, 2013 Fair Value Level 1	Jun. 30, 2014 Fair Value Level 1 6.95% Senior Notes	Dec. 31, 2013 Fair Value Level 1 6.95% Senior Notes	Jun. 30, 2014 Fair Value Level 2 6.7% Senior Notes	Dec. 31, 2013 Fair Value Level 2 6.7% Senior Notes	Jun. 30, 2014 Fair Value U.S. Treasury Notes Level 1	Dec. 31, 2013 Fair Value U.S. Treasury Notes Level 1	Jun. 30, 2014 Book Value	Dec. 31, 2013 Book Value	Jun. 30, 2014 Book Value 6.95% Senior Notes	Dec. 31, 2013 Book Value 6.95% Senior Notes	Jun. 30, 2014 Book Value 6.7% Senior Notes	Dec. 31, 2013 Book Value 6.7% Senior Notes	Jun. 30, 2014 Book Value U.S. Treasury Notes	Dec. 31, 2013 Book Value U.S. Treasury Notes	Jun. 30, 2014 6.7% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash and cash equivalents	$ 404,058	$ 342,065	$ 467,421	$ 378,358	$ 404,058	$ 342,065							$ 404,058	$ 342,065
Short-term investments	40,035	50,104									40,035	50,104							40,035	50,104
Long-term debt							$ 348,703	$ 309,852	$ 550,727	$ 507,697					$ 342,000	$ 342,000	$ 532,583	$ 532,449
Fair value assumption, interest rate																					6.59%