Nature of Business and Accounting Policies - Business (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) segment $ / shares	Dec. 31, 2014 USD ($) $ / shares	Dec. 31, 2013 USD ($) $ / shares
Valuation and Qualifying Accounts Disclosure [Line Items]
Number of operating segments | segment									1
Net loss	$ (74,669)	$ (93,816)	$ (220,992)	$ (198,704)	$ (180,846)	$ (170,060)	$ (159,382)	$ (232,457)	$ (588,181)	$ (742,745)	$ (687,550)
Basic EPS (usd per share) | $ / shares									$ (2.31)	$ (3.14)	$ (1.98)
Cash, cash equivalents and marketable securities	$ 1,040,000								$ 1,040,000
Accumulated Deficit
Valuation and Qualifying Accounts Disclosure [Line Items]
Net loss									$ (556,300)	$ (738,555)	$ (445,028)