Retirement Plans - Actuarial Assumptions Used for Pension Plans & Postretirement Medical Plan (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plans
Weighted Average Assumptions Used to Determine Benefit Obligations at December 31
Discount rate	3.70%	4.10%	3.80%
Rate of compensation increase	4.60%	4.50%	5.00%
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost for the Years Ended December 31
Discount rate	4.10%	3.80%	4.60%
Expected return on plan assets	7.40%	7.50%	7.50%
Rate of compensation increase	4.50%	5.00%	4.40%
Postretirement Medical Plan
Weighted Average Assumptions Used to Determine Benefit Obligations at December 31
Discount rate	3.50%	3.50%	3.10%
Initial health care trend rate	7.70%	6.70%	6.80%
Ultimate trend rate	4.50%	4.50%	4.50%
Year in which ultimate trend rate is reached	2038	2038	2029