Cash, Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Cash, Cash Equivalents and Marketable Securities
Cash and cash equivalents	$ 41,916	$ 146,470	$ 12,769	$ 18,130
Available-for-sale securities	100,085	49,897
Unrealized losses	0
U.S. Treasury securities
Cash, Cash Equivalents and Marketable Securities
Available-for-sale securities	50,000	$ 49,900
U.S. government sponsored entity securities
Cash, Cash Equivalents and Marketable Securities
Available-for-sale securities	$ 50,100