Fair Value of Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Minimum	Mar. 31, 2013 Maximum	Mar. 31, 2013 3.375% Senior Notes due September 15, 2021	Sep. 30, 2011 3.375% Senior Notes due September 15, 2021	Mar. 31, 2013 1.5% Convertible Senior Notes due February 15, 2024	Dec. 31, 2012 1.5% Convertible Senior Notes due February 15, 2024	Feb. 29, 2004 1.5% Convertible Senior Notes due February 15, 2024	Mar. 31, 2013 5.625% Municipal Bonds	Mar. 31, 2013 Fair Value Measurements Nonrecurring Carrying Value Level 1	Dec. 31, 2012 Fair Value Measurements Nonrecurring Carrying Value Level 1	Mar. 31, 2013 Fair Value Measurements Nonrecurring Carrying Value Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Carrying Value Level 2	Mar. 31, 2013 Fair Value Measurements Nonrecurring Carrying Value Level 3	Dec. 31, 2012 Fair Value Measurements Nonrecurring Carrying Value Level 3	Mar. 31, 2013 Fair Value Measurements Nonrecurring Carrying Value 3.375% Senior Notes due September 15, 2021 Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Carrying Value 3.375% Senior Notes due September 15, 2021 Level 2	Mar. 31, 2013 Fair Value Measurements Nonrecurring Carrying Value 1.5% Convertible Senior Notes due February 15, 2024 Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Carrying Value 1.5% Convertible Senior Notes due February 15, 2024 Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Carrying Value 5.625% Municipal Bonds Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Carrying Value Notes payable, including noncurrent portion Level 3	Mar. 31, 2013 Fair Value Measurements Nonrecurring Fair Value Level 1	Dec. 31, 2012 Fair Value Measurements Nonrecurring Fair Value Level 1	Mar. 31, 2013 Fair Value Measurements Nonrecurring Fair Value Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Fair Value Level 2	Mar. 31, 2013 Fair Value Measurements Nonrecurring Fair Value Level 3	Dec. 31, 2012 Fair Value Measurements Nonrecurring Fair Value Level 3	Mar. 31, 2013 Fair Value Measurements Nonrecurring Fair Value 3.375% Senior Notes due September 15, 2021 Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Fair Value 3.375% Senior Notes due September 15, 2021 Level 2	Mar. 31, 2013 Fair Value Measurements Nonrecurring Fair Value 1.5% Convertible Senior Notes due February 15, 2024 Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Fair Value 1.5% Convertible Senior Notes due February 15, 2024 Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Fair Value 5.625% Municipal Bonds Level 2	Dec. 31, 2012 Fair Value Measurements Nonrecurring Fair Value Notes payable, including noncurrent portion Level 3
Estimated fair values of the company's financial instruments that are not measured at fair value on a recurring basis
Cash											$ 1,366,670	$ 1,343,866											$ 1,366,670	$ 1,343,866
Cash equivalents													636,885	796,218											636,885	796,218
Marketable securities, current	160,442	137,127											55,067	34,688											55,067	34,688
Notes receivable, including noncurrent portion															34,408	34,471											34,408	34,471
Liabilities, Senior Notes																	496,274	496,164											523,783	527,219
Liabilities, Convertible Senior Notes							18,472	18,472											18,472	18,472											44,386	39,392
Liabilities, Municipal Bonds																					17,795												17,878
Debt instrument interest rate (as a percent)	5.625%				3.375%	3.375%	1.50%		1.50%	5.625%
Notes payable, including noncurrent portion																						$ 8,566												$ 8,566
Notes payable, maturity period			1 year	4 years