Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified					3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2013 Minimum	Mar. 31, 2013 Maximum	Mar. 31, 2013 Fair Value Level 1	Dec. 31, 2012 Fair Value Level 1	Mar. 31, 2013 Fair Value Level 1 Retail	Dec. 31, 2012 Fair Value Level 1 Retail	Mar. 31, 2013 Fair Value Level 2 Institutional and Other	Dec. 31, 2012 Fair Value Level 2 Institutional and Other	Mar. 31, 2013 Fair Value Government Backed Securities Level 1	Dec. 31, 2012 Fair Value Government Backed Securities Level 1	Mar. 31, 2013 Book Value	Dec. 31, 2012 Book Value	Mar. 31, 2013 Book Value Retail	Dec. 31, 2012 Book Value Retail	Mar. 31, 2013 Book Value Institutional and Other	Dec. 31, 2012 Book Value Institutional and Other	Mar. 31, 2013 Book Value Government Backed Securities	Dec. 31, 2012 Book Value Government Backed Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 766,700	$ 740,481	$ 639,130	$ 563,275			$ 766,700	$ 740,481							$ 766,700	$ 740,481
Short-term investments	125,595	115,700											125,595	115,700							125,595	115,700
Long-term investments	40,142	50,305											40,175	50,339							40,142	50,305
Long-term debt									$ 1,230,244	$ 1,238,204	$ 591,696	$ 589,435					$ 1,178,250	$ 1,178,250	$ 538,369	$ 538,657
Long term investment maturities					Nov. 30, 2014	Dec. 15, 2014
Fair value assumption, interest rate					0.00%	6.05%