Derivatives and Fair Value Measurements - Additional Information (Detail) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative [Line Items]
Natural gas portfolios, minimum % rate	25.00%
Natural gas portfolios, maximum % rate	35.00%
Retirement of long-term debt	$ 130,269,000	$ 377,457,000	$ 179,855,000	$ 432,865,000
February 2011 [Member]
Derivative [Line Items]
Debt maturing date	February 2011		February 2011
Retirement of long-term debt	200,000,000
May 2012 [Member]
Derivative [Line Items]
Debt maturing date	May 2012		May 2012
Retirement of long-term debt	200,000,000
Senior Notes, 4.45%, Due 2020 [Member]
Derivative [Line Items]
Notes payable	125,000,000		125,000,000
Debt instrument interest rate	4.45%		4.45%
Issuance of fixed rate debt (in years)	10 years
Notional amount	100,000,000		100,000,000
Senior Notes, 3.875%, Due 2022 [Member]
Derivative [Line Items]
Notes payable	$ 250,000,000		$ 250,000,000
Debt instrument interest rate	3.875%		3.875%
Issuance of fixed rate debt (in years)	10 years
Minimum [Member]
Derivative [Line Items]
Maturities of natural gas swaps	Oct. 01, 2013
Maximum [Member]
Derivative [Line Items]
Maturities of natural gas swaps	Mar. 01, 2015