Benefit Plans - Fair Value Measurement (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investments:
Percent of Total Investments	100.00%	100.00%
Investments	$ 206,033	$ 188,807
Equity securities
Investments:
Percent of Total Investments	43.00%	41.00%
Investments	$ 88,250	$ 76,578
Corporate bonds
Investments:
Percent of Total Investments	21.00%	22.00%
Investments	$ 43,152	$ 41,786
Mutual funds
Investments:
Percent of Total Investments	19.00%	19.00%
Investments	$ 38,975	$ 36,052
U.S. Treasury securities
Investments:
Percent of Total Investments	12.00%	12.00%
Investments	$ 24,910	$ 22,130
Cash and money market funds
Investments:
Percent of Total Investments	5.00%	6.00%
Investments	$ 10,402	$ 11,913
U.S. government agency securities
Investments:
Investments	344	348
Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Investments	162,537	126,756
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Investments:
Investments	88,250	76,578
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Investments:
Investments	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Investments:
Investments	38,975	36,052
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Investments:
Investments	24,910	2,213
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and money market funds
Investments:
Investments	10,402	11,913
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. government agency securities
Investments:
Investments	0	0
Significant Other Observable Inputs (Level 2)
Investments:
Investments	43,496	42,134
Significant Other Observable Inputs (Level 2) | Equity securities
Investments:
Investments	0	0
Significant Other Observable Inputs (Level 2) | Corporate bonds
Investments:
Investments	43,152	41,786
Significant Other Observable Inputs (Level 2) | Mutual funds
Investments:
Investments	0	0
Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Investments:
Investments	0	0
Significant Other Observable Inputs (Level 2) | Cash and money market funds
Investments:
Investments	0	0
Significant Other Observable Inputs (Level 2) | U.S. government agency securities
Investments:
Investments	344	348
Significant Unobservable Inputs (Level 3)
Investments:
Investments	0	0
Significant Unobservable Inputs (Level 3) | Equity securities
Investments:
Investments	0	0
Significant Unobservable Inputs (Level 3) | Corporate bonds
Investments:
Investments	0	0
Significant Unobservable Inputs (Level 3) | Mutual funds
Investments:
Investments	0	0
Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Investments:
Investments	0	0
Significant Unobservable Inputs (Level 3) | Cash and money market funds
Investments:
Investments	0	0
Significant Unobservable Inputs (Level 3) | U.S. government agency securities
Investments:
Investments	$ 0	$ 0