Investments Continuous Unrealized Loss Position (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	$ 154,192,000	$ 153,346,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	112,543,000	29,613,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	91,000	72,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	112,543,000	29,613,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	91,000	72,000
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	154,124,000	153,328,000
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Amortized Cost Basis	55,721,000	31,408,000
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Fair Value	55,719,000	31,396,000
Total, amortized cost basis	209,913,000	184,754,000
Total, fair value	209,843,000	184,724,000
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,850,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,850,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	0
Total, fair value	17,847,000	9,996,000
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		1,440,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses		1,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		1,440,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses		1,000
Total, fair value	0	1,440,000
Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	110,693,000	28,173,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	91,000	71,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	110,693,000	28,173,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	91,000	71,000
Total, fair value	$ 182,879,000	$ 162,669,000