Fair Value Measurements (Details) (USD $)	6 Months Ended										3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Convertible senior notes	Dec. 31, 2012 Convertible senior notes	Jun. 30, 2013 Unrestricted	Dec. 31, 2012 Unrestricted	Dec. 31, 2012 Measured on a recurring basis Fair Value	Dec. 31, 2012 Measured on a recurring basis Quoted Prices in Active Markets for Identical Assets (Level 1)	Dec. 31, 2012 Measured on a recurring basis Significant Other Observable Inputs (Level 2)	Jun. 30, 2013 Measured on a recurring basis Fair Value, Inputs, Level 3	Jun. 30, 2012 Measured on a recurring basis Fair Value, Inputs, Level 3	Jun. 30, 2013 Measured on a recurring basis Fair Value, Inputs, Level 3	Jun. 30, 2012 Measured on a recurring basis Fair Value, Inputs, Level 3	Jun. 30, 2013 Measured on a recurring basis Unrestricted Fair Value	Dec. 31, 2012 Measured on a recurring basis Unrestricted Fair Value	Jun. 30, 2013 Measured on a recurring basis Unrestricted Quoted Prices in Active Markets for Identical Assets (Level 1)	Dec. 31, 2012 Measured on a recurring basis Unrestricted Quoted Prices in Active Markets for Identical Assets (Level 1)	Jun. 30, 2013 Measured on a recurring basis Unrestricted Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 Measured on a recurring basis Unrestricted Significant Other Observable Inputs (Level 2)	Jun. 30, 2013 U.S. government obligations Unrestricted	Dec. 31, 2012 U.S. government obligations Unrestricted	Jun. 30, 2013 U.S. government obligations Measured on a recurring basis Unrestricted	Dec. 31, 2012 U.S. government obligations Measured on a recurring basis Unrestricted	Jun. 30, 2013 U.S. government obligations Measured on a recurring basis Unrestricted Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 U.S. government obligations Measured on a recurring basis Unrestricted Significant Other Observable Inputs (Level 2)	Jun. 30, 2013 Corporate bond Unrestricted	Jun. 30, 2013 Corporate bond Measured on a recurring basis Unrestricted	Jun. 30, 2013 Corporate bond Measured on a recurring basis Unrestricted Significant Other Observable Inputs (Level 2)	Jun. 30, 2013 Commercial paper Unrestricted	Jun. 30, 2013 Commercial paper Measured on a recurring basis Unrestricted	Jun. 30, 2013 Commercial paper Measured on a recurring basis Unrestricted Significant Other Observable Inputs (Level 2)	Jun. 30, 2013 Municipal bonds Unrestricted	Dec. 31, 2012 Municipal bonds Unrestricted	Jun. 30, 2013 Municipal bonds Measured on a recurring basis Unrestricted	Dec. 31, 2012 Municipal bonds Measured on a recurring basis Unrestricted	Jun. 30, 2013 Municipal bonds Measured on a recurring basis Unrestricted Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 Municipal bonds Measured on a recurring basis Unrestricted Significant Other Observable Inputs (Level 2)	Jun. 30, 2013 Foreign Government Debt Securities [Member] Unrestricted	Jun. 30, 2013 Foreign Government Debt Securities [Member] Measured on a recurring basis Unrestricted	Jun. 30, 2013 Foreign Government Debt Securities [Member] Measured on a recurring basis Unrestricted Significant Other Observable Inputs (Level 2)	Jun. 30, 2013 Equity securities Unrestricted	Dec. 31, 2012 Equity securities Unrestricted	Jun. 30, 2013 Equity securities Measured on a recurring basis Unrestricted	Dec. 31, 2012 Equity securities Measured on a recurring basis Unrestricted	Jun. 30, 2013 Equity securities Measured on a recurring basis Unrestricted Quoted Prices in Active Markets for Identical Assets (Level 1)	Dec. 31, 2012 Equity securities Measured on a recurring basis Unrestricted Quoted Prices in Active Markets for Identical Assets (Level 1)	Dec. 31, 2012 U.S. government obligations Restricted	Dec. 31, 2012 U.S. government obligations Measured on a recurring basis Restricted	Dec. 31, 2012 U.S. government obligations Measured on a recurring basis Restricted Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payments to Acquire Marketable Securities	$ 282,643,000	$ 270,907,000									$ 0	$ 0	$ 0	$ 0
Sales or maturities of marketable securities	307,244,000	171,881,000									0	0	0	0
Unrealized Gain (Loss) on Investments											0	0	0	0
Total convertible senior notes - par				400,000,000	400,000,000
Interest rate, stated percentage				1.875%	1.875%
Fair value of the outstanding notes				1,117,500,000	843,200,000
Available-for-sale Securities, Fair Value Disclosure			$ 354,958,000			$ 329,159,000	$ 349,049,000	$ 354,958,000	$ 3,393,000	$ 351,565,000					$ 329,159,000	$ 349,049,000	$ 2,300,000	$ 3,393,000	$ 326,859,000	$ 345,656,000	$ 85,405,000	$ 328,146,000	$ 85,405,000	$ 328,146,000	$ 85,405,000	$ 328,146,000	$ 157,328,000	$ 157,328,000	$ 157,328,000	$ 62,159,000	$ 62,159,000	$ 62,159,000	$ 17,151,000	$ 17,510,000	$ 17,151,000	$ 17,510,000	$ 17,151,000	$ 17,510,000	$ 4,816,000	$ 4,816,000	$ 4,816,000	$ 2,300,000	$ 3,393,000	$ 2,300,000	$ 3,393,000	$ 2,300,000	$ 3,393,000	$ 5,909,000	$ 5,909,000	$ 5,909,000