Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Assets:
Investments	$ 2,788		$ 4,168
Mark-to-market energy assets		102
Investments in equity securities [Member]
Assets:
Investments	0		2,013
Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities		124	331
Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Investments in equity securities [Member]
Assets:
Investments			2,007
Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments		512
Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Investments - other [Member]
Assets:
Investments		2,276	2,161
Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Mark-to-market energy assets [Member]
Assets:
Mark-to-market energy assets		379
Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Mark-to-market energy assets, including call options [Member]
Assets:
Mark-to-market energy assets			210
Recurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities		0	0
Recurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Investments in equity securities [Member]
Assets:
Investments			2,007
Recurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments		0
Recurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Investments - other [Member]
Assets:
Investments		2,276	2,161
Recurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Mark-to-market energy assets [Member]
Assets:
Mark-to-market energy assets		0
Recurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Mark-to-market energy assets, including call options [Member]
Assets:
Mark-to-market energy assets			0
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities		124	331
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Investments in equity securities [Member]
Assets:
Investments			0
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments		0
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Investments - other [Member]
Assets:
Investments		0	0
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mark-to-market energy assets [Member]
Assets:
Mark-to-market energy assets		379
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mark-to-market energy assets, including call options [Member]
Assets:
Mark-to-market energy assets			210
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mark-to-market energy liabilities [Member]
Liabilities:
Mark-to-market energy liabilities		0	0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Investments in equity securities [Member]
Assets:
Investments			0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments		512
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Investments - other [Member]
Assets:
Investments		0	0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mark-to-market energy assets [Member]
Assets:
Mark-to-market energy assets		0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mark-to-market energy assets, including call options [Member]
Assets:
Mark-to-market energy assets			$ 0