Fair Value Measurements (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Assets measured at fair value on a recurring basis
Interest rate on convertible debt (as a percent)	2.625%
2 5/8 percent convertible subordinated notes
Assets measured at fair value on a recurring basis
Interest rate on convertible debt (as a percent)	2.625%
Significant Other Observable Inputs (Level 2) | 2 5/8 percent convertible subordinated notes
Assets measured at fair value on a recurring basis
Fair value of convertible notes	$ 165,300,000	$ 151,100,000
Recurring basis | Total
Assets measured at fair value on a recurring basis
Cash equivalents	77,126,000	58,892,000
Total assets	330,111,000	338,361,000
Recurring basis | Total | Corporate debt securities
Assets measured at fair value on a recurring basis
Available-for-sale securities	165,140,000	166,922,000
Recurring basis | Total | Debt securities issued by U.S. government agencies
Assets measured at fair value on a recurring basis
Available-for-sale securities	52,152,000	80,440,000
Recurring basis | Total | Debt securities issued by the U.S. Treasury
Assets measured at fair value on a recurring basis
Available-for-sale securities	8,490,000	2,356,000
Recurring basis | Total | Debt securities issued by states of the United States and political subdivisions of the states
Assets measured at fair value on a recurring basis
Available-for-sale securities	24,602,000	28,469,000
Recurring basis | Total | Equity securities
Assets measured at fair value on a recurring basis
Available-for-sale securities	2,601,000	1,282,000
Recurring basis | Quoted Prices in Active Markets (Level 1)
Assets measured at fair value on a recurring basis
Cash equivalents	77,126,000	55,893,000
Total assets	88,217,000	59,531,000
Recurring basis | Quoted Prices in Active Markets (Level 1) | Debt securities issued by the U.S. Treasury
Assets measured at fair value on a recurring basis
Available-for-sale securities	8,490,000	2,356,000
Recurring basis | Quoted Prices in Active Markets (Level 1) | Equity securities
Assets measured at fair value on a recurring basis
Available-for-sale securities	2,601,000	1,282,000
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets measured at fair value on a recurring basis
Cash equivalents		2,999,000
Total assets	241,894,000	278,830,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt securities
Assets measured at fair value on a recurring basis
Available-for-sale securities	165,140,000	166,922,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Debt securities issued by U.S. government agencies
Assets measured at fair value on a recurring basis
Available-for-sale securities	52,152,000	80,440,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Debt securities issued by states of the United States and political subdivisions of the states
Assets measured at fair value on a recurring basis
Available-for-sale securities	$ 24,602,000	$ 28,469,000