Capital Stock and Derivative Instruments - Schedule of Unamortized Value of Restricted Stock (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	$ 418
Unamortized fair-value - event dependent	5,049
Total unamortized dollar value	5,467	$ 3,500
2nd quarter 2018 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	71
3rd quarter 2018 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	71
4th quarter 2018 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	71
1st quarter 2019 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	66
2nd quarter 2019 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	36
3rd quarter 2019 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	36
4th quarter 2019 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	36
1st quarter 2020 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	$ 31