Firm Transportation Obligation - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, noncurrent	$ 10,705	$ 12,042
Canonsburg, PA Regional Office
Restructuring Cost and Reserve [Line Items]
Restructuring and related costs in 2016, undiscounted	2,700
Restructuring and related costs in 2017, undiscounted	2,700
Restructuring and related costs in 2018, undiscounted	2,700
Restructuring and related costs in 2019, undiscounted	2,700
Restructuring and related costs in 2020, undiscounted	2,700
Restructuring and related costs due 2020 - 2022, undiscounted	4,600
Accounts Payable and Accrued Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, current	2,800
Other Noncurrent Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, noncurrent	$ 10,700