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Summary of Significant Accounting Policies - Insurance (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
Accounting Policies [Abstract]  
Employer's liability claims subject to deductible per occurrence $ 1,000,000
Worker's compensation claims per occurrence 5,000,000
Auto liability insurance claims deductible 10,000,000
General liability insurance claims deductible 10,000,000
Employee health care benefit plans subject to deductible per claimant $ 400,000