Postretirement Benefits Other Than Pensions - Summary of Assumptions Used to Determine Benefit Obligations (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Benefit Obligation
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year	7.27%	7.70%
Rate to which the cost trend rate is assumed to decline (ultimate trend rate)	4.50%	4.50%
Year of ultimate rate achievement	2038	2029
Weighted average discount rate	4.27%	3.85%
Net Periodic Benefit Cost
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year	7.70%	7.90%	8.20%
Rate to which the cost trend rate is assumed to decline (ultimate trend rate)	4.50%	4.50%	4.50%
Year of ultimate rate achievement	2029	2029	2029
Weighted average discount rate	3.85%	4.64%	3.38%