Summary of Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012
Liabilities:
Derivative liabilities	$ 12,800
Carrying Amount
Assets:
Cash and cash equivalents	368,882
Carrying Amount | Interest rate swap agreements
Liabilities:
Derivative liabilities	12,530
Carrying Amount | Natural gas swap agreements
Liabilities:
Derivative liabilities	244
Carrying Amount | Bank debt
Liabilities:
Long term debt fair value	1,281,529
Carrying Amount | 5% Senior Notes
Liabilities:
Long term debt fair value	500,000
Fair Value
Assets:
Cash and cash equivalents	368,882
Fair Value | Interest rate swap agreements
Liabilities:
Derivative liabilities	12,530
Fair Value | Natural gas swap agreements
Liabilities:
Derivative liabilities	244
Fair Value | Bank debt
Liabilities:
Long term debt fair value	1,281,529
Fair Value | 5% Senior Notes
Liabilities:
Long term debt fair value	$ 512,190