Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets:
Investments	$ 4,902	$ 3,644
Quoted Prices in Active Markets (Level 1) | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	0	0
Significant Other Observable Inputs (Level 2) | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	773	433
Significant Unobservable Inputs (Level 3) | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities		0
Equity Securities [Member]
Assets:
Investments		18
Equity Securities [Member] | Quoted Prices in Active Markets (Level 1)
Assets:
Investments	21	18
Equity Securities [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments		0
Equity Securities [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments		0
Guaranteed Income Fund [Member] | Quoted Prices in Active Markets (Level 1)
Assets:
Investments	0	0
Guaranteed Income Fund [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0	0
Guaranteed Income Fund [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments	561	279
Investments - other | Quoted Prices in Active Markets (Level 1)
Assets:
Investments	4,320	3,347
Investments - other | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0	0
Investments - other | Significant Unobservable Inputs (Level 3)
Assets:
Investments	0	0
Mark-to-market energy assets, including put option | Quoted Prices in Active Markets (Level 1)
Assets:
Mark-to-market energy assets, including put option	0	0
Mark-to-market energy assets, including put option | Significant Other Observable Inputs (Level 2)
Assets:
Mark-to-market energy assets, including put option	823	153
Mark-to-market energy assets, including put option | Significant Unobservable Inputs (Level 3)
Assets:
Mark-to-market energy assets, including put option	0	0
Fair Value | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	773	433
Fair Value | Equity Securities [Member]
Assets:
Investments	21	18
Fair Value | Guaranteed Income Fund [Member]
Assets:
Investments	561	279
Fair Value | Investments - other
Assets:
Investments	4,320	3,347
Fair Value | Mark-to-market energy assets, including put option
Assets:
Mark-to-market energy assets, including put option	$ 823	$ 153