Convertible Senior Notes - Schedule of Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Amortization of debt discount and issuance costs	$ 22,826	$ 22,040
Capitalization of interest expense	(3,987)	(3,402)
Total interest expense	$ 18,839	$ 18,638	$ 18,525