Schedule of Market Values of Pension Plan Assets (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	$ 116,697	$ 106,469	$ 104,074
Equity Securities | U.S. Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,212	8,600
Equity Securities | U.S. Mid Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,852	2,740
Equity Securities | U.S. Small Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,626	2,725
Equity Securities | International Developed
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,181	6,508
Equity Securities | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,139	4,154
Fixed Income Securities | Core Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27,062	23,222
Fixed Income Securities | Long Duration Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	61,718	55,933
Fixed income securities | REIT Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,395	2,124
Cash | Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	512	463
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	116,697	106,469
Fair Value, Inputs, Level 2 | Equity Securities | U.S. Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,212	8,600
Fair Value, Inputs, Level 2 | Equity Securities | U.S. Mid Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,852	2,740
Fair Value, Inputs, Level 2 | Equity Securities | U.S. Small Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,626	2,725
Fair Value, Inputs, Level 2 | Equity Securities | International Developed
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,181	6,508
Fair Value, Inputs, Level 2 | Equity Securities | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,139	4,154
Fair Value, Inputs, Level 2 | Fixed Income Securities | Core Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27,062	23,222
Fair Value, Inputs, Level 2 | Fixed Income Securities | Long Duration Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	61,718	55,933
Fair Value, Inputs, Level 2 | Fixed income securities | REIT Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,395	2,124
Fair Value, Inputs, Level 2 | Cash | Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 512	$ 463