Employee Benefits - Weighted-Average Rates to Determine Net Postretirement Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.75%	3.50%	4.25%
Expected return on plan assets	6.00%	6.25%	7.25%
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.00%	3.75%	4.50%
Expected return on plan assets	4.50%	4.50%	5.25%