Postretirement Benefits - Pension Assets by Components at Fair Value (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Investment funds, unfunded commitments	$ 408,000
Pension plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 500,773	$ 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	4 years	5 years
Investment funds, unfunded commitments		$ 4,100
Pension plan | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 485,124	$ 456,496
Percentage of plan assets	97.00%	97.00%
Pension plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 242,555	$ 222,466
Pension plan | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	242,569	234,030
Pension plan | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension plan | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 15,649	$ 12,267
Percentage of plan assets	3.00%	3.00%
Pension plan | Total corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 212,055	$ 205,501
Percentage of total plan assets, within plan asset category	43.00%	44.00%
Pension plan | 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
Pension plan | Total corporate bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	212,055	205,501
Pension plan | 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
Pension plan | Financial
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 53,219	$ 51,111
Percentage of total plan assets, within plan asset category	11.00%	11.00%
Pension plan | 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
Pension plan | Financial | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	53,219	51,111
Pension plan | Financial | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Pension plan | Utilities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 45,364	$ 42,758
Percentage of total plan assets, within plan asset category	9.00%	9.00%
Pension plan | 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
Pension plan | Utilities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	45,364	42,758
Pension plan | Utilities | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Pension plan | Energy
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 21,100	$ 21,907
Percentage of total plan assets, within plan asset category	4.00%	5.00%
Pension plan | 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
Pension plan | Energy | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	21,100	21,907
Pension plan | Energy | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Pension plan | Other corporates
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 92,372	$ 89,725
Percentage of total plan assets, within plan asset category	19.00%	19.00%
Pension plan | 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
Pension plan | Other corporates | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	92,372	89,725
Pension plan | Other corporates | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Pension plan | Exchange traded funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 217,649	$ 207,176
Percentage of total plan assets, within plan asset category	43.00%	44.00%
Pension plan | 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	$ 217,649	$ 207,176
Pension plan | 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
Pension plan | 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
Pension plan | Other bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 264	$ 251
Percentage of total plan assets, within plan asset category	0.00%	0.00%
Pension plan | 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
Pension plan | Other bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	264	251
Pension plan | Other bonds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Pension plan | Guaranteed annuity contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 30,250	$ 28,278
Percentage of total plan assets, within plan asset category	6.00%	6.00%
Pension plan | 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
Pension plan | Guaranteed annuity contract | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	30,250	28,278
Pension plan | 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
Pension plan | Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 18,023	$ 8,414
Percentage of total plan assets, within plan asset category	4.00%	2.00%
Pension plan | 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	$ 18,023	$ 8,414
Pension plan | 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
Pension plan | 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
Pension plan | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 6,883	$ 6,876
Percentage of total plan assets, within plan asset category	1.00%	1.00%
Pension plan | 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	$ 6,883	$ 6,876
Pension plan | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	0	0
Pension plan | Other | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, within plan asset category	$ 0	$ 0