Fair Value Measurements (Schedule of Fair Value Measurement) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	$ 917,159	$ 893,251
Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments and cash equivalents	922,072	921,879
Notes Receivable, Fair Value Disclosure	18,882
Other Assets, Fair Value Disclosure	18,882
Obligations, Fair Value Disclosure		(1,200)
Liabilities, Fair Value Disclosure		(1,200)
Total Fair Value [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	413	22,255
Total Fair Value [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	7,775	7,473
Total Fair Value [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	7,492	9,482
Total Fair Value [Member] | U.S. Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	716,310	692,995
Total Fair Value [Member] | U.S. Government Agencies Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	190,082	189,674
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments and cash equivalents	8,188	29,728
Notes Receivable, Fair Value Disclosure	0
Other Assets, Fair Value Disclosure	0
Obligations, Fair Value Disclosure		0
Liabilities, Fair Value Disclosure		0
Level 1 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	413	22,255
Level 1 [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	7,775	7,473
Level 1 [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	0	0
Level 1 [Member] | U.S. Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	0	0
Level 1 [Member] | U.S. Government Agencies Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	0	0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments and cash equivalents	913,884	892,151
Notes Receivable, Fair Value Disclosure	0
Other Assets, Fair Value Disclosure	0
Obligations, Fair Value Disclosure		0
Liabilities, Fair Value Disclosure		0
Level 2 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 2 [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	0	0
Level 2 [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	7,492	9,482
Level 2 [Member] | U.S. Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	716,310	692,995
Level 2 [Member] | U.S. Government Agencies Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	190,082	189,674
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments and cash equivalents	0	0
Notes Receivable, Fair Value Disclosure	18,882
Other Assets, Fair Value Disclosure	18,882
Obligations, Fair Value Disclosure		(1,200)
Liabilities, Fair Value Disclosure		(1,200)
Level 3 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 3 [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	0	0
Level 3 [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	0	0
Level 3 [Member] | U.S. Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	0	0
Level 3 [Member] | U.S. Government Agencies Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate Fair value	$ 0	$ 0