Capital Stock and Derivative Instruments - Schedule of Unamortized Value of Restricted Stock (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	$ 516
Unamortized fair-value - event dependent	3,022
Total unamortized dollar value	3,538	$ 7,700	$ 10,000
1st quarter 2018 [Member]
Amortization Expense [Line Items]
Unamortized fair value subject to vesting schedule	98
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