Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Level 1 [Member]
Assets
Total assets	$ 57,086	$ 14,950
Liabilities
Convertible notes payable	0	0
Level 1 [Member] | Debt securities of U.S. government agencies [Member]
Assets
Total assets	0	0
Level 1 [Member] | Money market funds [Member]
Assets
Total assets	57,086	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	298,967	186,137
Liabilities
Convertible notes payable	411,255	0
Level 2 [Member] | Debt securities of U.S. government agencies [Member]
Assets
Total assets	22,537	0
Level 2 [Member] | Money market funds [Member]
Assets
Total assets	0	0
Level 2 [Member] | U.S. Treasury securities [Member]
Assets
Total assets	21,694
Level 2 [Member] | Government-backed security [Member]
Assets
Total assets	22,000	20,000
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	55,463	28,924
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total assets	177,273	137,213
Level 3 [Member]
Assets
Total assets	0	0
Liabilities
Convertible notes payable	0	0
Level 3 [Member] | Debt securities of U.S. government agencies [Member]
Assets
Total assets	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