Postretirement Benefits - Pension Assets by Components at Fair Value (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Investment funds, unfunded commitments	$ 456,000
Pension plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 478,180	$ 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	$ 463,412	$ 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	$ 225,880	$ 222,466
Pension plan | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	237,532	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	$ 14,768	$ 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	$ 208,715	$ 205,501
Percentage of total plan assets, within plan asset category	44.00%	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	208,715	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	$ 52,297	$ 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	52,297	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	$ 43,921	$ 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	43,921	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	$ 20,497	$ 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	20,497	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	$ 92,000	$ 89,725
Percentage of total plan assets, within plan asset category	20.00%	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	92,000	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	$ 199,512	$ 207,176
Percentage of total plan assets, within plan asset category	42.00%	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	$ 199,512	$ 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	$ 266	$ 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	266	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	$ 28,551	$ 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	28,551	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	$ 19,992	$ 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	$ 19,992	$ 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	$ 6,376	$ 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	$ 6,376	$ 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