Employee Benefits - Schedule of Investments Measured Using Net Asset Value Per Share Practical Expedient (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	100.00%	100.00%
Investments, Fair Value	$ 51,660	$ 44,437
Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	50.90%	52.70%
Investments, Fair Value	$ 26,274	$ 23,398
Collective Funds [Member] | Equity [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	19.30%	20.80%
Investments, Fair Value	$ 9,932	$ 9,204
Collective Funds [Member] | Diversified [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent		0.00%
Collective Funds [Member] | US Debt Securities [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	15.20%	12.10%
Investments, Fair Value	$ 7,874	$ 5,386
Collective Funds [Member] | International Equities [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	10.10%	9.70%
Investments, Fair Value	$ 5,208	$ 4,311
Limited Partnerships [Member] | Emerging Markets [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	3.20%	2.90%
Investments, Fair Value	$ 1,635	$ 1,289
Limited Partnerships [Member] | Multi-strategy [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	1.20%	1.90%
Investments, Fair Value	$ 644	$ 826
Hedge Funds [Member] | Multi-strategy [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent		3.60%
Investments, Fair Value		$ 1,593
Hedge Funds [Member] | Global Opportunities [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	0.50%	0.30%
Investments, Fair Value	$ 259	$ 150
Hedge Funds [Member] | Private Investment Entities and/or Separately Managed Accounts [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	1.40%	1.40%
Investments, Fair Value	$ 722	$ 639