Benefit Plans - Fair Value Measurement (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of Total Investments	100.00%	100.00%
Investments:
Investments, Fair Value Disclosure	$ 188,807	$ 193,994
Equity securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of Total Investments	41.00%	41.00%
Investments:
Investments, Fair Value Disclosure	$ 76,578	$ 80,174
Corporate bonds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of Total Investments	22.00%	22.00%
Investments:
Investments, Fair Value Disclosure	$ 41,786	$ 42,750
Mutual funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of Total Investments	19.00%	18.00%
Investments:
Investments, Fair Value Disclosure	$ 36,052	$ 35,709
U.S. Treasury securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of Total Investments	12.00%	11.00%
Investments:
Investments, Fair Value Disclosure	$ 22,130	$ 21,604
Cash and money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of Total Investments	6.00%	7.00%
Investments:
Investments, Fair Value Disclosure	$ 11,913	$ 12,895
U.S. government agency securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of Total Investments		1.00%
Investments:
Investments, Fair Value Disclosure	348	$ 862
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Investments:
Investments, Fair Value Disclosure	146,673	150,382
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Equity securities [Member]
Investments:
Investments, Fair Value Disclosure	76,578	80,174
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	36,052	35,709
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury securities [Member]
Investments:
Investments, Fair Value Disclosure	22,130	21,604
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash and money market funds [Member]
Investments:
Investments, Fair Value Disclosure	11,913	12,895
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. government agency securities [Member]
Investments:
Investments, Fair Value Disclosure	0	0
Significant Other Observable Inputs (Level 2) [Member]
Investments:
Investments, Fair Value Disclosure	42,134	43,612
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	41,786	42,750
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	348	862
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