Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Assets:
Commodity derivative assets – current	$ 13,680	$ 3,830
Commodity derivative assets – noncurrent	8,572	1,552
Liabilities:
Commodity derivative liabilities – current	(1,045)	(10,141)
Commodity derivative liabilities – noncurrent	0	0
Fair Value, Measurements, Recurring | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Assets of SERP	4,021	3,734
Liabilities:
Deferred compensation – SERP obligations	(4,125)	(3,879)
Fair Value, Measurements, Recurring | Level 1
Assets:
Assets of SERP	4,021	3,734
Liabilities:
Deferred compensation – SERP obligations	(4,125)	(3,879)
Fair Value, Measurements, Recurring | Level 2
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation – SERP obligations	0	0
Fair Value, Measurements, Recurring | Level 3
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation – SERP obligations	0	0
Fair Value, Measurements, Recurring | Commodity contracts | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets:
Commodity derivative assets – current	13,680	3,830
Commodity derivative assets – noncurrent	8,572	1,552
Liabilities:
Commodity derivative liabilities – current	(1,045)	(10,141)
Commodity derivative liabilities – noncurrent	0
Fair Value, Measurements, Recurring | Commodity contracts | Level 1
Assets:
Commodity derivative assets – current	0	0
Commodity derivative assets – noncurrent	0	0
Liabilities:
Commodity derivative liabilities – current	0	0
Commodity derivative liabilities – noncurrent	0
Fair Value, Measurements, Recurring | Commodity contracts | Level 2
Assets:
Commodity derivative assets – current	13,680	3,830
Commodity derivative assets – noncurrent	8,572	1,552
Liabilities:
Commodity derivative liabilities – current	(1,045)	(10,141)
Commodity derivative liabilities – noncurrent	0
Fair Value, Measurements, Recurring | Commodity contracts | Level 3
Assets:
Commodity derivative assets – current	0	0
Commodity derivative assets – noncurrent	0	0
Liabilities:
Commodity derivative liabilities – current	0	0
Commodity derivative liabilities – noncurrent	$ 0