Pensions and Other Benefits - Summary of Pension Plan Asset Allocation and Current Weighted Average Policy Range (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Current asset allocation target	100.00%
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan asset allocation percentages	100.00%	100.00%
Cash and cash equivalents
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Current asset allocation target	0.50%
Cash and cash equivalents | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan asset allocation percentages	1.40%	1.10%
Fixed income
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Current asset allocation target	29.50%
Fixed income | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan asset allocation percentages	26.10%	21.40%
Public equity
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Current asset allocation target	46.00%
Public equity | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan asset allocation percentages	53.30%	53.80%
Real estate and infrastructure
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Current asset allocation target	12.00%
Real estate and infrastructure | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan asset allocation percentages	6.20%	7.50%
Absolute return
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Current asset allocation target	12.00%
Absolute return | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan asset allocation percentages	13.00%	16.20%
Maximum [Member] | Cash and cash equivalents | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.05
Maximum [Member] | Fixed income | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.4
Maximum [Member] | Public equity | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.55
Maximum [Member] | Real estate and infrastructure | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.2
Maximum [Member] | Absolute return | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.18
Minimum [Member] | Cash and cash equivalents | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0
Minimum [Member] | Fixed income | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.2
Minimum [Member] | Public equity | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0.35
Minimum [Member] | Real estate and infrastructure | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	.04
Minimum [Member] | Absolute return | Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Target asset allocation percentages	0