Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financial liabilities:
Financial liabilities fair value	$ 130.8	$ 93.5
Milestone Rights Liability
Financial liabilities:
Financial liabilities fair value	19.0	19.1
Carrying Value
Financial liabilities:
Financial liabilities fair value	114.2	85.9
Carrying Value | Milestone Rights Liability
Financial liabilities:
Financial liabilities fair value	8.8	8.8
Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	130.8	93.5
Estimate of Fair Value Measurement | Milestone Rights Liability | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	19.0	19.1
5.75% Senior convertible notes due October 23, 2021
Financial liabilities:
Financial liabilities fair value	19.7	19.8
5.75% Senior convertible notes due October 23, 2021 | Carrying Value
Financial liabilities:
Financial liabilities fair value	19.1	24.4
5.75% Senior convertible notes due October 23, 2021 | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	19.7	19.8
Facility Financing Obligation
Financial liabilities:
Financial liabilities fair value	11.4	54.6
Facility Financing Obligation | Carrying Value
Financial liabilities:
Financial liabilities fair value	14.2	52.7
Facility Financing Obligation | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	11.4	$ 54.6
Note Payable to Related Party
Financial liabilities:
Financial liabilities fair value	80.7
Note Payable to Related Party | Carrying Value
Financial liabilities:
Financial liabilities fair value	72.1
Note Payable to Related Party | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	$ 80.7