Benefit Plans - Fair Value Measurement (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	100.00%	100.00%
Investments:
Investments, Fair Value Disclosure	$ 138,331	$ 95,437
Equity securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	49.00%	49.00%
Investments:
Investments, Fair Value Disclosure	67,653	46,446
U.S. Treasury securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	10.00%	17.00%
Investments:
Investments, Fair Value Disclosure	13,249	16,418
Corporate bonds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	20.00%	12.00%
Investments:
Investments, Fair Value Disclosure	28,153	11,743
Mutual funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	10.00%	11.00%
Investments:
Investments, Fair Value Disclosure	13,545	10,360
Cash and money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	9.00%	7.00%
Investments:
Investments, Fair Value Disclosure	12,355	6,354
U.S. government agency securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	1.00%	2.00%
Investments:
Investments, Fair Value Disclosure	1,615	2,421
Trust preferred securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Investments	1.00%	2.00%
Investments:
Investments, Fair Value Disclosure	1,761	1,695
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Investments:
Investments, Fair Value Disclosure	108,563	81,273
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Equity securities [Member]
Investments:
Investments, Fair Value Disclosure	67,653	46,446
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury securities [Member]
Investments:
Investments, Fair Value Disclosure	13,249	16,418
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mutual funds [Member]
Investments:
Investments, Fair Value Disclosure	13,545	10,360
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash and money market funds [Member]
Investments:
Investments, Fair Value Disclosure	12,355	6,354
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Trust preferred securities [Member]
Investments:
Investments, Fair Value Disclosure	1,761	1,695
Significant Other Observable Inputs (Level 2) [Member]
Investments:
Investments, Fair Value Disclosure	29,768	14,164
Significant Other Observable Inputs (Level 2) [Member] | Corporate bonds [Member]
Investments:
Investments, Fair Value Disclosure	28,153	11,743
Significant Other Observable Inputs (Level 2) [Member] | U.S. government agency securities [Member]
Investments:
Investments, Fair Value Disclosure	$ 1,615	$ 2,421