Accounting for Derivative Instruments and Hedging Activities - Derivative Volumes Expected to Settle (Detail)	Dec. 31, 2016 MMBTU
Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	32,800,000
Natural Gas Contract Expected to be Settled in Year 2017 [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contract Expected to be Settled in Year 2017 [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	26,000,000
Natural Gas Contract Expected to be Settled in Year 2018 [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contract Expected to be Settled in Year 2018 [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	6,800,000