Fair Value (Details) - Fair value, recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Assets
Cash equivalents	$ 258	$ 733
Total marketable securities	429,051	417,126
Total financial assets	429,309	417,859
Money market funds [Member]
Assets
Cash equivalents	258	733
Commercial paper and corporate bonds [Member]
Assets
Total marketable securities	414,551	417,126
U.S. government agency debt securities [Member]
Assets
Total marketable securities	14,500	0
Level 1 [Member]
Assets
Cash equivalents	258	733
Total marketable securities	0	0
Total financial assets	258	733
Level 1 [Member] | Money market funds [Member]
Assets
Cash equivalents	258	733
Level 1 [Member] | Commercial paper and corporate bonds [Member]
Assets
Total marketable securities	0	0
Level 1 [Member] | U.S. government agency debt securities [Member]
Assets
Total marketable securities	0	0
Level 2 [Member]
Assets
Cash equivalents	0	0
Total marketable securities	429,051	417,126
Total financial assets	429,051	417,126
Level 2 [Member] | Money market funds [Member]
Assets
Cash equivalents	0	0
Level 2 [Member] | Commercial paper and corporate bonds [Member]
Assets
Total marketable securities	414,551	417,126
Level 2 [Member] | U.S. government agency debt securities [Member]
Assets
Total marketable securities	$ 14,500	$ 0