Fair Value of Financial Instruments -Cash and Cash Equivalents, Available-for-sale Securities, Restricted Cash Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and cash equivalents	$ 42,118	$ 34,885	$ 25,804	$ 24,548
Amortized cost	125,635	118,538
Unrealized gains	41	8
Unrealized losses	(5)	0
Fair value	125,671	118,546
Restricted cash- cash	0	114
Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and cash equivalents	25,434	23,419
Amortized cost	42,117	34,883
Unrealized gains	1	2
Unrealized losses	0	0
Fair value	42,118	34,885
Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	58,728	83,541
Unrealized gains	22	6
Unrealized losses	(2)	0
Fair value	58,748	83,547
Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	24,790
Unrealized gains	18
Unrealized losses	(3)
Fair value	24,805
Restricted Cash
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Restricted cash- cash		114
Money Market Funds
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized gains	0	0
Unrealized losses	0	0
Money Market Funds | Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	5,086	170
Fair value	5,086	170
Commercial Paper | Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	11,597	11,294
Unrealized gains	1	2
Unrealized losses	0	0
Fair value	11,598	11,296
Commercial Paper | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	43,960	21,078
Unrealized gains	14	2
Unrealized losses	(1)	0
Fair value	43,973	21,080
Corporate bonds | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	9,640
Unrealized gains	7
Unrealized losses	0
Fair value	9,647
Corporate bonds | Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	16,083
Unrealized gains	13
Unrealized losses	(2)
Fair value	16,094
Municipal bonds | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	1,127
Unrealized gains	0
Unrealized losses	(1)
Fair value	1,126
US Government Agencies | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	4,001	62,463
Unrealized gains	1	4
Unrealized losses	0	0
Fair value	4,002	62,467
US Government Agencies | Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	4,711
Unrealized gains	2
Unrealized losses	0
Fair value	4,713
Asset backed securities | Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	3,996
Unrealized gains	3
Unrealized losses	(1)
Fair value	$ 3,998