Employee Benefit Plans - Pension Plan Asset Allocations (Detail)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Plan Assets	100.00%	100.00%
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan Assets	70.00%	65.00%
Cash and equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan Assets	1.00%	2.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Plan Assets	29.00%	33.00%