Postretirement Benefits - Pension Assets by Components at Fair Value (Detail) - Pension plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 597,547	$ 529,532	$ 468,763
Percentage of plan assets	100.00%	100.00%
Investment funds, percentage ownership by pension plan	1.00%	1.00%
Expected life of investment	3 years	4 years
Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 582,398	$ 513,219
Percentage of plan assets	97.00%	97.00%
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 339,839	$ 273,239
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	242,559	239,980
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 15,149	$ 16,313
Percentage of plan assets	3.00%	3.00%
Total corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 207,577	$ 209,603
Percentage of total plan assets, within plan asset category	35.00%	40.00%
Total corporate bonds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 0	$ 0
Total corporate bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	207,577	209,603
Total corporate bonds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Financial
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 52,522	$ 52,252
Percentage of total plan assets, within plan asset category	9.00%	10.00%
Financial | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 0	$ 0
Financial | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	52,522	52,252
Financial | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Utilities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 43,663	$ 45,888
Percentage of total plan assets, within plan asset category	7.00%	9.00%
Utilities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 0	$ 0
Utilities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	43,663	45,888
Utilities | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Energy
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 22,719	$ 22,480
Percentage of total plan assets, within plan asset category	4.00%	4.00%
Energy | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 0	$ 0
Energy | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	22,719	22,480
Energy | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Other corporates
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 88,673	$ 88,983
Percentage of total plan assets, within plan asset category	15.00%	17.00%
Other corporates | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 0	$ 0
Other corporates | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	88,673	88,983
Other corporates | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Exchange traded funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 315,720	$ 245,170
Percentage of total plan assets, within plan asset category	52.00%	46.00%
Exchange traded funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 315,720	$ 245,170
Exchange traded funds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Exchange traded funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Other bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 239	$ 258
Percentage of total plan assets, within plan asset category	0.00%	0.00%
Other bonds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 0	$ 0
Other bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	239	258
Other bonds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Guaranteed annuity contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 34,743	$ 30,119
Percentage of total plan assets, within plan asset category	6.00%	6.00%
Guaranteed annuity contract | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 0	$ 0
Guaranteed annuity contract | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	34,743	30,119
Guaranteed annuity contract | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 13,731	$ 20,960
Percentage of total plan assets, within plan asset category	2.00%	4.00%
Short-term investments | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 13,731	$ 20,960
Short-term investments | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Short-term investments | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 10,388	$ 7,109
Percentage of total plan assets, within plan asset category	2.00%	1.00%
Other | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 10,388	$ 7,109
Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Other | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 0	$ 0