Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financial liabilities:
Financial liabilities fair value	$ 102.0	$ 93.5
Carrying Value
Financial liabilities:
Financial liabilities fair value	111.4	86.0
Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	102.0	93.5
2021 Notes
Financial liabilities:
Financial liabilities fair value	17.5	19.8
2021 Notes | Carrying Value
Financial liabilities:
Financial liabilities fair value	19.1	24.4
2021 Notes | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	17.5	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	11.3	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
Milestone Rights Liability
Financial liabilities:
Financial liabilities fair value	18.1	19.1
Milestone Rights Liability | Carrying Value
Financial liabilities:
Financial liabilities fair value	8.9	8.9
Milestone Rights Liability | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	18.1	$ 19.1
Note Payable to Related Party
Financial liabilities:
Financial liabilities fair value	55.0
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	$ 55.0