Capital Stock and Derivative Instruments - Schedule of Unamortized Value of Restricted Stock (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	$ 747
Unamortized fair-value - event dependent	4,764
Total unamortized dollar value	5,511	$ 3,500
3rd quarter 2018 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	109
4th quarter 2018 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	109
1st quarter 2019 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	103
2nd quarter 2019 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	73
3rd quarter 2019 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	72
4th quarter 2019 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	72
1st quarter 2020 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	67
2nd quarter 2020 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	37
3rd quarter 2020 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	37
4th quarter 2020 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	37
1st quarter 2021 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	$ 31