Retirement Plans and Other Post-Retirement Benefits - Schedule of Targeted Range of Investment Allocations of Plan Assets (Detail)	Dec. 31, 2017
Domestic Equity [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation for the plan assets	35.00%
Domestic Equity [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation for the plan assets	45.00%
International Equity [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation for the plan assets	8.00%
International Equity [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation for the plan assets	14.00%
Real Estate Investment Trusts (REIT) [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation for the plan assets	2.00%
Real Estate Investment Trusts (REIT) [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation for the plan assets	6.00%
Fixed Income, Cash and Cash Equivalents [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation for the plan assets	55.00%
Fixed Income, Cash and Cash Equivalents [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation for the plan assets	35.00%