Postretirement Benefits Other Than Pensions - Weighted Average Assumptions Used to Determine Net Postretirement Benefit Expense (Detail) (Other Postretirement Benefit Plan [Member])	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Postretirement Benefit Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	3.94%	3.08%	3.92%
Measurement date	2014-12-31	2013-12-31	2012-12-31