Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets:
Investments	$ 5,212	$ 4,902
Equity Securities [Member]
Assets:
Investments	22	21
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Assets, Fair Value Disclosure	4,647	4,341
Quoted Prices in Active Markets (Level 1) [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities including natural gas futures	0	0
Quoted Prices in Active Markets (Level 1) [Member] | Equity Securities [Member]
Assets:
Investments	22	21
Quoted Prices in Active Markets (Level 1) [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments	0	0
Quoted Prices in Active Markets (Level 1) [Member] | Investments - other [Member]
Assets:
Investments	4,625	4,320
Quoted Prices in Active Markets (Level 1) [Member] | Total Investments [Member]
Assets:
Investments	4,647	4,341
Quoted Prices in Active Markets (Level 1) [Member] | Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Mark-to-market energy assets	0	0
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Assets, Fair Value Disclosure	1,339	823
Significant Other Observable Inputs (Level 2) [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities including natural gas futures	189	773
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] | Investments - other [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] | Total Investments [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] | Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Mark-to-market energy assets	1,339	823
Significant Unobservable Inputs (Level 3) [Member]
Assets:
Assets, Fair Value Disclosure	565	561
Significant Unobservable Inputs (Level 3) [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities including natural gas futures	0	0
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]
Assets:
Investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments	565	561
Significant Unobservable Inputs (Level 3) [Member] | Investments - other [Member]
Assets:
Investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Total Investments [Member]
Assets:
Investments	565	561
Significant Unobservable Inputs (Level 3) [Member] | Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Mark-to-market energy assets	0	0
Recurring [Member]
Assets:
Assets, Fair Value Disclosure	6,551	5,725
Recurring [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities including natural gas futures	189	773
Recurring [Member] | Equity Securities [Member]
Assets:
Investments	22	21
Recurring [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments	565	561
Recurring [Member] | Investments - other [Member]
Assets:
Investments	4,625	4,320
Recurring [Member] | Total Investments [Member]
Assets:
Investments	5,212	4,902
Recurring [Member] | Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Mark-to-market energy assets	$ 1,339	$ 823