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Employee Benefit Plans - Schedule of Aggregate Fair Value of Asset and Supplemental Asset Mix (Detail) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Combined Master Retirement Trust Asset [Line Items]    
CMRT asset value $ 637.8 $ 648.8
CMRT assets comprised of:    
Assets not subject to fair value hierarchy 30.00% 30.00%
Assets subject to fair value hierarchy:    
Fair value input 100.00% 100.00%
CMRT asset mix:    
Percentage of combined master retirement trust asset mix 100.00% 100.00%
Domestic equities, principally publicly traded    
CMRT asset mix:    
Percentage of combined master retirement trust asset mix 31.00% 29.00%
International equities, principally publicly traded    
CMRT asset mix:    
Percentage of combined master retirement trust asset mix 22.00% 22.00%
Fixed income securities, principally publicly traded    
CMRT asset mix:    
Percentage of combined master retirement trust asset mix 36.00% 38.00%
Privately managed limited partnerships    
CMRT asset mix:    
Percentage of combined master retirement trust asset mix 5.00% 5.00%
Hedge funds    
CMRT asset mix:    
Percentage of combined master retirement trust asset mix 5.00% 5.00%
Other, primarily cash    
CMRT asset mix:    
Percentage of combined master retirement trust asset mix 1.00% 1.00%
Level 1    
Assets subject to fair value hierarchy:    
Fair value input 54.00% 54.00%
Level 2    
Assets subject to fair value hierarchy:    
Fair value input 8.00% 7.00%
Level 3    
Assets subject to fair value hierarchy:    
Fair value input 8.00% 9.00%