Other Post-Employment Benefits and Other Employee Benefit Plans - Narrative (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Other Post-Employment Benefits and Other Employee Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Minimum age of employee for entitlement of other post retirement benefit plans	55 years
Minimum number of years of service for entitlement of other post retirement benefit plans	10 years
Accumulated post-retirement benefit obligations, weighted-average discount rate	3.40%	3.90%
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for the year	6.80%
Health care cost trend rate downgraded	5.00%
Canada Plans
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for the year	5.00%