Employee Benefits - Fair Value of Plan Assets and Major Categories (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	100.00%	100.00%
Total, fair value of plan assets	$ 33,812	$ 32,322
Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	15,658	13,292
Fair Value Measurements, Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	15,794	16,774
Fair Value Measurements, Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	2,360	2,256	1,691
Collective Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	53.40%	52.90%
Total, fair value of plan assets	18,069	17,092
Collective Funds [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	2,757	856
Collective Funds [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	15,312	16,236
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	23.10%	25.90%
Total, fair value of plan assets	7,797	8,355
Equity Securities [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	7,797	8,355
Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	15.50%	13.10%
Total, fair value of plan assets	5,244	4,250
Mutual Funds [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	4,762	3,832
Mutual Funds [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	482	418
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	7.00%	7.00%
Total, fair value of plan assets	2,360	2,256
Hedge Funds [Member] | Fair Value Measurements, Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	2,360	2,256
Short-Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	1.00%	1.10%
Total, fair value of plan assets	342	369
Short-Term Investments [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	342	249
Short-Term Investments [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets		$ 120