Weighted-Average Assumptions Used to Determine Net Periodic Benefit Expense (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Net Periodic Benefit Costs Weighted Average Assumptions [Line Items]
Expected return on plan assets	5.40%
Pension Plans [Member]
Schedule Of Net Periodic Benefit Costs Weighted Average Assumptions [Line Items]
Discount rate	3.89%	4.04%	3.78%
Expected return on plan assets	5.40%	6.10%	5.80%
Rate of compensation increase	3.72%	3.74%	3.76%
Other Retirement Plans [Member]
Schedule Of Net Periodic Benefit Costs Weighted Average Assumptions [Line Items]
Discount rate	3.85%	4.00%	3.65%