Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial liabilities:
Financial liabilities fair value	$ 93.5	$ 123.2
Warrant Liability
Financial liabilities:
Financial liabilities fair value	0.0	7.4	$ 0.0
Milestone Rights Liability
Financial liabilities:
Financial liabilities fair value	19.1	18.4
Carrying Value
Financial liabilities:
Financial liabilities fair value	86.0	115.2
Carrying Value | Warrant Liability
Financial liabilities:
Financial liabilities fair value		7.4
Carrying Value | Milestone Rights Liability
Financial liabilities:
Financial liabilities fair value	8.9	8.9
Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	93.5	123.2
Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3 | Warrant Liability
Financial liabilities:
Financial liabilities fair value		7.4
Estimate of Fair Value Measurement | Milestone Rights Liability | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	19.1	18.4
5.75% Senior convertible notes due October 23, 2021
Financial liabilities:
Financial liabilities fair value	19.8
5.75% Senior convertible notes due October 23, 2021 | Carrying Value
Financial liabilities:
Financial liabilities fair value	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	19.8
5.75% Senior convertible notes due August 15, 2018
Financial liabilities:
Financial liabilities fair value		22.9
5.75% Senior convertible notes due August 15, 2018 | Carrying Value
Financial liabilities:
Financial liabilities fair value		27.6
5.75% Senior convertible notes due August 15, 2018 | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value		22.9
2019 Notes and Tranche B Notes
Financial liabilities:
Financial liabilities fair value	54.6	74.5
2019 Notes and Tranche B Notes | Carrying Value
Financial liabilities:
Financial liabilities fair value	52.7	71.3
2019 Notes and Tranche B Notes | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 3
Financial liabilities:
Financial liabilities fair value	$ 54.6	$ 74.5