Employee Benefit Plans - Asset Allocation Ranges (Detail)	Dec. 31, 2017
Minimum | Cash
Defined Benefit Plan Disclosure [Line Items]
Minimum %	0.00%
Minimum | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Minimum %	15.00%
Minimum | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Minimum %	30.00%
Maximum | Cash
Defined Benefit Plan Disclosure [Line Items]
Minimum %	5.00%
Maximum | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Minimum %	75.00%
Maximum | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Minimum %	80.00%