Postretirement Benefits - Pension Assets by Components at Fair Value (Detail) - Funded defined benefit pension - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 455,735	$ 392,672
Total percentage	100.00%	100.00%
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 206,372	$ 170,822
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	249,363	221,850
Financial
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 51,045	$ 44,236
Total percentage	11.00%	11.00%
Financial | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 51,045	$ 44,236
Utilities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 43,401	$ 39,443
Total percentage	10.00%	10.00%
Utilities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 43,401	$ 39,443
Energy
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 21,919	$ 19,744
Total percentage	5.00%	5.00%
Energy | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 21,919	$ 19,744
Other corporates
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 92,421	$ 83,202
Total percentage	20.00%	22.00%
Other corporates | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 92,421	$ 83,202
Total corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 208,786	$ 186,625
Total percentage	46.00%	48.00%
Total corporate bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 208,786	$ 186,625
Exchange traded funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 189,103	$ 157,717
Total percentage	41.00%	40.00%
Exchange traded funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 189,103	$ 157,717
Other bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 258	$ 245
Total percentage	0.00%	0.00%
Other bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 258	$ 245
Other long-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 11,959	$ 8,475
Total percentage	3.00%	2.00%
Other long-term investments | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 11,959	$ 8,475
Guaranteed annuity contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 28,360	$ 26,505
Total percentage	6.00%	7.00%
Guaranteed annuity contract | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 28,360	$ 26,505
Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 10,107	$ 9,289
Total percentage	2.00%	2.00%
Short-term investments | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 10,107	$ 9,289
Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 7,162	$ 3,816
Total percentage	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	$ 7,162	$ 3,816