Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified																					3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2014 Fair Value Level 1	Dec. 31, 2013 Fair Value Level 1	Mar. 31, 2014 Fair Value Level 1 Retail	Dec. 31, 2013 Fair Value Level 1 Retail	Mar. 31, 2014 Fair Value Level 2 Institutional and Other	Dec. 31, 2013 Fair Value Level 2 Institutional and Other	Mar. 31, 2014 Fair Value U.S. Treasury Notes Level 1	Dec. 31, 2013 Fair Value U.S. Treasury Notes Level 1	Mar. 31, 2014 Book Value	Dec. 31, 2013 Book Value	Mar. 31, 2014 Book Value Retail	Dec. 31, 2013 Book Value Retail	Mar. 31, 2014 Book Value Institutional and Other	Dec. 31, 2013 Book Value Institutional and Other	Mar. 31, 2014 Book Value U.S. Treasury Notes	Dec. 31, 2013 Book Value U.S. Treasury Notes	Mar. 31, 2014 Institutional and Other Minimum	Mar. 31, 2014 Institutional and Other Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash and cash equivalents	$ 872,910	$ 830,014	$ 766,700	$ 740,481	$ 872,910	$ 830,014							$ 872,910	$ 830,014
Short-term investments	40,056	50,104									40,056	50,104							40,056	50,104
Long-term debt							$ 1,170,617	$ 1,048,010	$ 536,843	$ 512,635					$ 1,178,250	$ 1,178,250	$ 537,408	$ 537,454
Fair value assumption, interest rate																					0.00%	6.91%