Retirement Plans - Schedule of Weighted-Average Assumptions Used to Determine Net Periodic Cost (Detail) - Plans Net Periodic Cost [Member]	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Periodic Benefit Cost Assumptions [Line Items]
Discount rate	3.88%	3.61%	4.01%
Health care cost trend: Initial	6.25%	6.50%	6.75%
Health care cost trend: Ultimate	4.75%	4.75%	4.75%
Year ultimate reached	2022	2022	2022
Corridor	10.00%	10.00%	10.00%
Recognition period for gains and losses	11 years	11 years	10 years