Annual report pursuant to Section 13 and 15(d)

Retirement Plans and Related Post Retirement Benefits - Assumptions Used to Calculate the Post Retirement Benefit Obligation (Detail)

v3.8.0.1
Retirement Plans and Related Post Retirement Benefits - Assumptions Used to Calculate the Post Retirement Benefit Obligation (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
SERP      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount Rate 3.60% 4.20%  
Future Average Compensation Increases     5.00%
SERP Medical      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount Rate 3.60% 4.20%  
Future Average Compensation Increases 5.50% 5.72% 5.32%
Minimum | SERP      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Future Average Compensation Increases 2.00% 3.00%  
Future Average Compensation Increases 3.00% 3.00%  
Maximum | SERP      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Future Average Compensation Increases 3.00% 5.00%  
Future Average Compensation Increases 5.00% 5.00%