Consolidated Statements of Comprehensive Loss (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Unrealized gain (loss) on available-for-sale securities, tax	$ 9.7	$ 2.2	$ 0.0