Capital Stock and Derivative Instruments - Schedule of Vesting Shares Subject to Repurchase (Detail) - Restricted shares subject to repurchase [Member] - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amortization Expense [Line Items]
Unvested shares	4,053,640	4,039,591	4,197,396	1,343,764
1st quarter 2018 [Member]
Amortization Expense [Line Items]
Unvested shares	219,128
1st quarter 2019 [Member]
Amortization Expense [Line Items]
Unvested shares	167,848
4th quarter 2019 [Member]
Amortization Expense [Line Items]
Unvested shares	3,555,556
1st quarter 2020 [Member]
Amortization Expense [Line Items]
Unvested shares	111,108