Benefit Plans - Fair Value Measurement (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	100.00%	100.00%
Investments:
Investments, Fair Value Disclosure	$ 193,994	$ 184,206
Equity securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	41.00%	42.00%
Investments:
Investments, Fair Value Disclosure	80,174	77,565
Corporate bonds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	22.00%	23.00%
Investments:
Investments, Fair Value Disclosure	42,750	41,863
Mutual funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	18.00%	18.00%
Investments:
Investments, Fair Value Disclosure	35,709	33,690
U.S. Treasury securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	11.00%	10.00%
Investments:
Investments, Fair Value Disclosure	21,604	19,057
Cash and money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	7.00%	5.00%
Investments:
Investments, Fair Value Disclosure	12,895	9,939
U.S. government agency securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	1.00%	1.00%
Investments:
Investments, Fair Value Disclosure	862	1,896
Trust preferred securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments		1.00%
Investments:
Investments, Fair Value Disclosure		196
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Investments:
Investments, Fair Value Disclosure	150,382	140,447
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Equity securities [Member]
Investments:
Investments, Fair Value Disclosure	80,174	77,565
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate bonds [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mutual funds [Member]
Investments:
Investments, Fair Value Disclosure	35,709	33,690
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury securities [Member]
Investments:
Investments, Fair Value Disclosure	21,604	19,057
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash and money market funds [Member]
Investments:
Investments, Fair Value Disclosure	12,895	9,939
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. government agency securities [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Trust preferred securities [Member]
Investments:
Investments, Fair Value Disclosure		196
Significant Other Observable Inputs (Level 2) [Member]
Investments:
Investments, Fair Value Disclosure	43,612	43,759
Significant Other Observable Inputs (Level 2) [Member] | Equity securities [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Other Observable Inputs (Level 2) [Member] | Corporate bonds [Member]
Investments:
Investments, Fair Value Disclosure	42,750	41,863
Significant Other Observable Inputs (Level 2) [Member] | Mutual funds [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury securities [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Other Observable Inputs (Level 2) [Member] | Cash and money market funds [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Other Observable Inputs (Level 2) [Member] | U.S. government agency securities [Member]
Investments:
Investments, Fair Value Disclosure	862	1,896
Significant Other Observable Inputs (Level 2) [Member] | Trust preferred securities [Member]
Investments:
Investments, Fair Value Disclosure		0
Significant Unobservable Inputs (Level 3) [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Unobservable Inputs (Level 3) [Member] | Equity securities [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate bonds [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Unobservable Inputs (Level 3) [Member] | Mutual funds [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury securities [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Unobservable Inputs (Level 3) [Member] | Cash and money market funds [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Unobservable Inputs (Level 3) [Member] | U.S. government agency securities [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Unobservable Inputs (Level 3) [Member] | Trust preferred securities [Member]
Investments:
Investments, Fair Value Disclosure		$ 0