Employee Benefits - Pension Plan Assets Measured at Fair Value on Recurring Basis (Detail) (Fair Value Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	$ 93,486	$ 89,556
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	5,073	2,946
Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	26,726	26,448
Equity Index Mutual Funds-Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value		1,558
Equity Index Mutual Funds-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	3,728	2,497
Domestic Individual Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	57,085	55,206
International Individual Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	874	901
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	88,413	86,610
Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0	0
Level 1 [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	26,726	26,448
Level 1 [Member] | Equity Index Mutual Funds-Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value		1,558
Level 1 [Member] | Equity Index Mutual Funds-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	3,728	2,497
Level 1 [Member] | Domestic Individual Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	57,085	55,206
Level 1 [Member] | International Individual Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	874	901
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	5,073	2,946
Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	5,073	2,946
Level 2 [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0	0
Level 2 [Member] | Equity Index Mutual Funds-Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value		0
Level 2 [Member] | Equity Index Mutual Funds-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0	0
Level 2 [Member] | Domestic Individual Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0	0
Level 2 [Member] | International Individual Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0	0
Level 3 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0	0
Level 3 [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0	0
Level 3 [Member] | Equity Index Mutual Funds-Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value		0
Level 3 [Member] | Equity Index Mutual Funds-International [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0	0
Level 3 [Member] | Domestic Individual Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0	0
Level 3 [Member] | International Individual Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	$ 0	$ 0