Fair Value of Financial Instruments -Cash and Cash Equivalents, Available-for-sale Securities, Restricted Cash Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and cash equivalents	$ 68,619	$ 34,885	$ 28,459	$ 24,548
Amortized cost	126,467	118,538
Unrealized gains	21	8
Unrealized losses	(36)	0
Fair value	126,452	118,546
Restricted cash- cash	0	114
Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and cash equivalents	26,338	23,419
Amortized cost	68,620	34,883
Unrealized gains	1	2
Unrealized losses	(2)	0
Fair value	68,619	34,885
Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	37,750	83,541
Unrealized gains	15	6
Unrealized losses	(7)	0
Fair value	37,758	83,547
Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	20,097
Unrealized gains	5
Unrealized losses	(27)
Fair value	20,075
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	22,972	170
Fair value	22,972	170
Commercial Paper | Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	17,596	11,294
Unrealized gains	1	2
Unrealized losses	0	0
Fair value	17,597	11,296
Commercial Paper | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	28,255	21,078
Unrealized gains	13	2
Unrealized losses	0	0
Fair value	28,268	21,080
Corporate bonds | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	8,341
Unrealized gains	2
Unrealized losses	(7)
Fair value	8,336
Corporate bonds | Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	14,373
Unrealized gains	4
Unrealized losses	(23)
Fair value	14,354
Municipal bonds | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	1,154
Unrealized gains	0
Unrealized losses	0
Fair value	1,154
US Government Agencies | Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost		62,463
Unrealized gains		4
Unrealized losses		0
Fair value		62,467
US Government Agencies | Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	4,724
Unrealized gains	1
Unrealized losses	0
Fair value	4,725
Asset backed securities | Long-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost	1,000
Unrealized gains	0
Unrealized losses	(4)
Fair value	$ 996