Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Level 1 [Member]
Assets
Total assets	$ 49,321	$ 14,950
Liabilities
Convertible notes payable	0	0
Level 1 [Member] | Money market funds [Member]
Assets
Total assets	49,321	14,950
Level 1 [Member] | U.S. Treasury securities [Member]
Assets
Total assets		0
Level 1 [Member] | Government-backed security [Member]
Assets
Total assets	0	0
Level 1 [Member] | Asset-backed securities [Member]
Assets
Total assets	0	0
Level 1 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	0	0
Level 2 [Member]
Assets
Total assets	377,364	186,137
Liabilities
Convertible notes payable	317,021	0
Level 2 [Member] | U.S. agency debt securities [Member]
Assets
Total assets	35,871
Level 2 [Member] | Money market funds [Member]
Assets
Total assets	0	0
Level 2 [Member] | U.S. Treasury securities [Member]
Assets
Total assets	21,791	0
Level 2 [Member] | Government-backed security [Member]
Assets
Total assets	39,500	20,000
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	62,557	28,924
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	217,645	137,213
Level 3 [Member]
Assets
Total assets	0	0
Liabilities
Convertible notes payable	0	0
Level 3 [Member] | Money market funds [Member]
Assets
Total assets	0	0
Level 3 [Member] | U.S. Treasury securities [Member]
Assets
Total assets		0
Level 3 [Member] | Government-backed security [Member]
Assets
Total assets	0	0
Level 3 [Member] | Asset-backed securities [Member]
Assets
Total assets	0	0
Level 3 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	$ 0	$ 0