Defined Contribution Plans, Pension and Other Post-Retirement Benefit Plans - Fair Values of Pension Plan Assets by Asset Category and by Level (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	$ 80,423	$ 69,470
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	264	174
U.S. large value
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	5,499	4,800
U.S. large growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	5,792	4,805
International blend
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	10,734	7,954
Emerging markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	3,613	2,464
Balanced
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	21,895	18,486
Government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	9,806	8,402
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	12,667	12,976
U.S. property
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	10,153	9,409
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	15,168	12,243
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	264	174
Level 1 | U.S. large value
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	5,499	4,800
Level 1 | U.S. large growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	5,792	4,805
Level 1 | Emerging markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	3,613	2,464
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	55,102	47,818
Level 2 | International blend
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	10,734	7,954
Level 2 | Balanced
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	21,895	18,486
Level 2 | Government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	9,806	8,402
Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	12,667	12,976
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	10,153	9,409	$ 8,645
Level 3 | U.S. property
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plans assets	$ 10,153	$ 9,409