Assets and Liabilities Measured at Fair Value on Recurring Basis Based on Fair Value Hierarchy (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liabilities	$ (0.8)
Common stock warrant liability, estimated fair value	(7.6)	$ (11.1)	$ (5.6)	$ (5.9)	$ (8.3)	$ (9.3)
Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net derivative liabilities	(0.8)
Fair Value, Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	0.6
Derivative liabilities	(1.4)
Fair Value, Recurring | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Common stock warrant liability, estimated fair value	$ (7.6)		$ (5.6)