Derivative and Other Financial Instruments and Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis Based on Fair Value Hierarchy (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring Basis
Common stock warrant liability, estimated fair value	$ (1.2)	$ (2.1)	$ (4.4)	$ (4.2)	$ (6.1)	$ (6.9)
Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis
Net derivative assets (liabilities)	0.2		0.2
Fair Value, Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis
Derivative assets	0.2		0.6
Derivative liabilities as classified in the unaudited condensed consolidated balance sheet			(0.4)
Fair Value, Recurring | Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis
Common stock warrant liability, estimated fair value	$ (1.2)		$ (4.4)