Retirement Plans - Weighted Average Assumptions Used to Determine Benefit Obligation (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Ultimate	0.00%	0.00%
Discount rate	3.80%	4.30%
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Ultimate	5.00%	4.90%
Discount rate	4.00%	4.20%
Pre-Age 65 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	0.00%	0.00%
Pre-Age 65 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	6.10%	6.10%
Post-Age 65 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	0.00%	0.00%
Post-Age 65 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	6.10%	6.10%