Variable Interest Entities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 160,203	$ 11,639	$ 12,381	$ 7,041
Other current assets	1,520us-gaap_OtherAssetsCurrent	959us-gaap_OtherAssetsCurrent
Capitalized IPO expenses	2,268	0
Total current assets	185,158us-gaap_AssetsCurrent	24,881us-gaap_AssetsCurrent
Other assets	207us-gaap_OtherAssetsNoncurrent	299us-gaap_OtherAssetsNoncurrent
Total assets	258,029us-gaap_Assets	104,713us-gaap_Assets
Accounts payable	7,698us-gaap_AccountsPayableCurrent	3,951us-gaap_AccountsPayableCurrent
Accrued liabilities	4,866us-gaap_AccruedLiabilitiesCurrent	5,337us-gaap_AccruedLiabilitiesCurrent
Current portion of notes payable	334us-gaap_NotesPayableCurrent	9,109us-gaap_NotesPayableCurrent
Total current liabilities	22,779us-gaap_LiabilitiesCurrent	28,939us-gaap_LiabilitiesCurrent
Long-term portion of notes payable (eliminates in consolidation)	195,908us-gaap_LongTermNotesPayable	0us-gaap_LongTermNotesPayable
Total liabilities	233,621us-gaap_Liabilities	55,100us-gaap_Liabilities
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Cash and cash equivalents	756	0
Other current assets	18
Capitalized IPO expenses	2,268
Total current assets	3,042
Other assets	1
Total assets	3,043
Accounts payable	2,211	0
Accrued liabilities	77
Current portion of notes payable	334	0
Total current liabilities	2,622
Long-term portion of notes payable (eliminates in consolidation)	2,331
Total liabilities	$ 4,953

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