Capital Stock and Derivative Instruments - Schedule of Vesting Shares Subject to Repurchase (Detail) - Restricted shares subject to repurchase [Member] - shares	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amortization Expense [Line Items]
Unvested shares	3,834,512	4,053,640	4,039,591
Total unvested shares - subject to vesting schedule	278,956
Unvested shares - event dependent	3,555,556
1st quarter 2019 [Member]
Amortization Expense [Line Items]
Unvested shares	167,848
1st quarter 2020 [Member]
Amortization Expense [Line Items]
Unvested shares	111,108