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Postretirement Plans - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2016
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Prior unrecognized gains included in Accumulated OCI $ 0.4
U.S. Postretirement Health Care Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Health care cost trend rate 4.50%
Increase in accumulated postretirement benefit obligation $ 0.6
Increase in service cost and interest cost components 0.1
Decrease in accumulated postretirement benefit obligation 0.6
Decrease in service cost and interest cost components $ 0.1
Pre-65 Health Care Cost Trend Rate [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Health care cost trend rate 5.90%
Post-65 Health Care Cost Trend Rate [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Health care cost trend rate 6.60%
Voluntary Employees Beneficiary Association Plan Assets [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Description of basis used to determine overall expected long-term, rate-of-return-on-assets assumption The objective of the VEBA trust is to achieve net of expense returns that meet or exceed the 6.6% long-term return on plan assets assumption used for reporting purposes.