Consolidated Statements of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Cash flows from operating activities:
Net income	$ 53,595	$ 180,644	$ 172,228
Pension and postretirement expense (benefit)		3,152	(10,203)
Deferred income taxes		4,911	4,871
As Previously Reported
Cash flows from operating activities:
Net income	41,238		153,652
Pension and postretirement expense (benefit)			19,070
Deferred income taxes			(5,826)
Effect of Accounting Change
Cash flows from operating activities:
Net income	12,357		18,576
Pension and postretirement expense (benefit)			(29,273)
Deferred income taxes			$ 10,697