Postretirement Benefits - Pension Assets by Components at Fair Value (Detail) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 377,624	$ 328,871	$ 307,596
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	$ 173,356	$ 149,580
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	204,268	179,291
Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 180,506	$ 160,036
Total percentage	48.00%	49.00%
Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 180,506	$ 160,036
Exchange Traded Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 164,351	$ 134,771
Total percentage	43.00%	41.00%
Exchange Traded Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 164,351	$ 134,771
Other Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 256	$ 258
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	$ 256	$ 258
Other long-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,304
Total percentage	1.00%
Other long-term investments | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,304
Guaranteed Annuity Contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 21,202	$ 18,997
Total percentage	6.00%	6.00%
Guaranteed Annuity Contract | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 21,202	$ 18,997
Short-term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 3,984	$ 7,391
Total percentage	1.00%	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	$ 3,984	$ 7,391
Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 5,021	$ 7,418
Total percentage	1.00%	2.00%
Other | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 5,021	$ 7,418
Financial | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 43,451	$ 41,578
Total percentage	12.00%	13.00%
Financial | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 43,451	$ 41,578
Utilities | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 46,144	$ 43,890
Total percentage	12.00%	13.00%
Utilities | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 46,144	$ 43,890
Energy | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 25,023	$ 25,427
Total percentage	7.00%	8.00%
Energy | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 25,023	$ 25,427
Other corporate sectors | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 65,888	$ 49,141
Total percentage	17.00%	15.00%
Other corporate sectors | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 65,888	$ 49,141