Summary of Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Liabilities:
Derivative liabilities	$ 13,800
Carrying Amount
Assets:
Cash and cash equivalents	422,525
Carrying Amount | Interest rate swap agreements
Liabilities:
Derivative liabilities	13,812
Carrying Amount | Natural gas swap agreements
Liabilities:
Derivative liabilities	13
Carrying Amount | Bank debt
Liabilities:
Long term debt fair value	1,489,701
Carrying Amount | 5% Senior Notes
Liabilities:
Long term debt fair value	500,000
Fair Value
Assets:
Cash and cash equivalents	422,525
Fair Value | Interest rate swap agreements
Liabilities:
Derivative liabilities	13,812
Fair Value | Natural gas swap agreements
Liabilities:
Derivative liabilities	13
Fair Value | Bank debt
Liabilities:
Long term debt fair value	1,489,701
Fair Value | 5% Senior Notes
Liabilities:
Long term debt fair value	$ 523,750