Summary of Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Liabilities:
Derivative liabilities	$ 11,200
Carrying Amount
Assets:
Cash and cash equivalents	158,637
Carrying Amount | Natural gas swap agreements
Assets:
Derivative Assets	204
Carrying Amount | Interest rate swap agreements
Liabilities:
Derivative liabilities	11,357
Carrying Amount | Bank debt
Liabilities:
Long term debt fair value	1,283,649
Carrying Amount | 5% Senior Notes
Liabilities:
Long term debt fair value	500,000
Fair Value
Assets:
Cash and cash equivalents	158,637
Fair Value | Natural gas swap agreements
Assets:
Derivative Assets	204
Fair Value | Interest rate swap agreements
Liabilities:
Derivative liabilities	11,357
Fair Value | Bank debt
Liabilities:
Long term debt fair value	1,283,649
Fair Value | 5% Senior Notes
Liabilities:
Long term debt fair value	$ 517,475