Cash, Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Cash, Cash Equivalents and Marketable Securities
Cash and cash equivalents	$ 61,695	$ 61,695	$ 146,470	$ 1,200	$ 18,130
Available-for-sale securities	99,984	99,984	$ 49,897
Unrealized losses		0
Net realized gains or losses	$ 0	$ 0