Pensions and Other Benefits - Summary of Pension Plan Asset Allocation and Current Weighted Average Policy Range (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Asset allocation target	100.00%	100.00%
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of plan assets	100.00%	100.00%
Cash and cash equivalents
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Asset allocation target	1.20%	0.50%
Cash and cash equivalents | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of plan assets	1.10%	1.40%
Cash and cash equivalents | Minimum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0	0
Cash and cash equivalents | Maximum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.1	0.05
Fixed income
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Asset allocation target	24.10%	29.50%
Fixed income | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of plan assets	25.60%	26.10%
Fixed income | Minimum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.2	0.2
Fixed income | Maximum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.4	0.4
Public equity
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Asset allocation target	45.10%	46.00%
Public equity | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of plan assets	50.20%	53.30%
Public equity | Minimum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.35	0.35
Public equity | Maximum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.55	0.55
Real estate and infrastructure
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Asset allocation target	9.80%	12.00%
Real estate and infrastructure | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of plan assets	7.70%	6.20%
Real estate and infrastructure | Minimum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.04	.04
Real estate and infrastructure | Maximum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.13	0.2
Private debt
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Asset allocation target	9.80%	0.00%
Private debt | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of plan assets	1.30%	0.00%
Private debt | Minimum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages		0
Private debt | Maximum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages		0
Absolute return
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Asset allocation target	10.00%	12.00%
Absolute return | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of plan assets	14.10%	13.00%
Absolute return | Minimum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.04	0
Absolute return | Maximum [Member] | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.13	0.18