Consolidated Statements of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from operating activities:
Net income	$ 89,516	$ 118,633
Pension and postretirement expense	1,582	243
Deferred income taxes	6,506	10,800
As Previously Reported
Cash flows from operating activities:
Net income		112,414
Pension and postretirement expense		10,024
Deferred income taxes		7,238
Effect of Accounting Change
Cash flows from operating activities:
Net income		6,219
Pension and postretirement expense		(9,781)
Deferred income taxes		$ 3,562