Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets:
Investments	$ 3,412	$ 3,678
Equity Securities [Member]
Assets:
Investments	18	0
Quoted Prices in Active Markets (Level 1) [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities	0	0
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] | Equity Securities [Member]
Assets:
Investments	18
Quoted Prices in Active Markets (Level 1) [Member] | Investments - other [Member]
Assets:
Investments	3,118	3,391
Quoted Prices in Active Markets (Level 1) [Member] | Mark To Market Energy Assets Including Put Option [Member]
Assets:
Mark-to-market energy assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities	154	1,018
Significant Other Observable Inputs (Level 2) [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Assets:
Investments	0
Significant Other Observable Inputs (Level 2) [Member] | Investments - other [Member]
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) [Member] | Mark To Market Energy Assets Including Put Option [Member]
Assets:
Mark-to-market energy assets	286	1,055
Significant Unobservable Inputs (Level 3) [Member] | Mark-to-market energy liabilities [Member]
Assets:
Mark-to-market energy assets	0	0	$ 0
Significant Unobservable Inputs (Level 3) [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments	276	287
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]
Assets:
Investments	0
Significant Unobservable Inputs (Level 3) [Member] | Investments - other [Member]
Assets:
Investments	0	0
Recurring [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities	154	1,018
Recurring [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments	276	287
Recurring [Member] | Equity Securities [Member]
Assets:
Investments	18
Recurring [Member] | Investments - other [Member]
Assets:
Investments	3,118	3,391
Recurring [Member] | Mark To Market Energy Assets Including Put Option [Member]
Assets:
Mark-to-market energy assets	$ 286	$ 1,055