Benefit Plans - Fair Value Measurement (Detail) - Pension Plan - Qualified Plan - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments:
Percent of total investments	100.15797%	100.00%
Investments	$ 221,131	$ 206,033
Equity securities
Investments:
Percent of total investments	38.00%	43.00%
Investments	$ 84,791	$ 88,250
Corporate bonds
Investments:
Percent of total investments	22.00%	21.00%
Investments	$ 47,471	$ 43,152
Mutual funds
Investments:
Percent of total investments	23.00%	19.00%
Investments	$ 48,814	$ 38,975
U.S. Treasury securities
Investments:
Percent of total investments	13.00%	12.00%
Investments	$ 28,671	$ 24,910
Cash and money market funds
Investments:
Percent of total investments	4.00%	5.00%
Investments	$ 9,522	$ 10,402
U.S. government agency securities
Investments:
Investments	1,862	344
Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Investments	171,798	162,537
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Investments:
Investments	84,791	88,250
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	48,814	38,975
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Investments:
Investments	28,671	24,910
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and money market funds
Investments:
Investments	9,522	10,402
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	49,333	43,496
Significant Other Observable Inputs (Level 2) | Equity securities
Investments:
Investments	0	0
Significant Other Observable Inputs (Level 2) | Corporate bonds
Investments:
Investments	47,471	43,152
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	1,862	344
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