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Employee Benefit Plans - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Mar. 31, 2018
Jun. 30, 2018
Retirement Benefits [Abstract]    
Employer discretionary contribution amount $ 7,000  
Unrecognized actuarial gains (losses), before tax   $ 56,222
Unrecognized actuarial gains (losses), net of tax   35,420
Amortization expected to be recognized   4,653
Amortization expected to be recognized, net of tax   $ 2,932