Fair Value of Financial Instruments - Cash and Cash Equivalents, Available-for-sale Securities Reconciliation (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	$ 144,945,000	$ 132,766,000
Unrealized gains	66,000	45,000
Unrealized losses	(16,000)	(7,000)
Fair value	144,995,000	132,804,000
Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	48,296,000	42,368,000
Unrealized gains	0	1,000
Unrealized losses	0	0
Fair value	48,296,000	42,369,000
Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	57,284,000	54,810,000
Unrealized gains	35,000	17,000
Unrealized losses	(7,000)	0
Fair value	57,312,000	54,827,000
Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	39,365,000	35,588,000
Unrealized gains	31,000	27,000
Unrealized losses	(9,000)	(7,000)
Fair value	39,387,000	35,608,000
Cash | Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	29,594,000	27,488,000
Fair value	29,594,000	27,488,000
Money Market Funds | Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	1,566,000	183,000
Unrealized gains	0	0
Unrealized losses	0	0
Fair value	1,566,000	183,000
Commercial Paper | Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	15,599,000	14,697,000
Unrealized gains	0	1,000
Unrealized losses	0	0
Fair value	15,599,000	14,698,000
Commercial Paper | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	22,974,000	32,784,000
Unrealized gains	7,000	6,000
Unrealized losses	0	0
Fair value	22,981,000	32,790,000
Corporate bonds | Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	1,537,000
Unrealized gains	0
Unrealized losses	0
Fair value	1,537,000
Corporate bonds | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	29,904,000	16,894,000
Unrealized gains	24,000	11,000
Unrealized losses	(7,000)	0
Fair value	29,921,000	16,905,000
Corporate bonds | Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	14,305,000	15,256,000
Unrealized gains	12,000	22,000
Unrealized losses	(7,000)	(4,000)
Fair value	14,310,000	15,274,000
Municipal Bonds | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost		1,128,000
Unrealized gains		0
Unrealized losses		0
Fair value		1,128,000
US Government Agencies | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	4,406,000	4,004,000
Unrealized gains	4,000	0
Unrealized losses	0	0
Fair value	4,410,000	4,004,000
US Government Agencies | Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	7,151,000	14,835,000
Unrealized gains	5,000	1,000
Unrealized losses	0	(3,000)
Fair value	7,156,000	14,833,000
Asset backed securities | Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	17,909,000	5,497,000
Unrealized gains	14,000	4,000
Unrealized losses	(2,000)	0
Fair value	$ 17,921,000	$ 5,501,000