Assets Measured at Fair Value on Recurring Basis for Pension Plan (Detail) - Pension Plans - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	$ 102,315	$ 91,058	$ 87,194
Mutual Fund
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	91,528	82,230
Mutual Fund | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	50,373	42,134
Mutual Fund | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	34,757	33,924
Mutual Fund | Equity And Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	6,398	6,172
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	1,200
Mutual Fund Including Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	92,728
Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	9,587	8,828
Fair Value, Inputs, Level 1 | Mutual Fund
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	91,528	82,230
Fair Value, Inputs, Level 1 | Mutual Fund | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	50,373	42,134
Fair Value, Inputs, Level 1 | Mutual Fund | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	34,757	33,924
Fair Value, Inputs, Level 1 | Mutual Fund | Equity And Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	6,398	$ 6,172
Fair Value, Inputs, Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	1,200
Fair Value, Inputs, Level 1 | Mutual Fund Including Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets measured at fair value	$ 92,728