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Retirement Benefit Obligations - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Change in Discount Rate	0.25%
Increase or decrease of Net Periodic Benefit Cost (NPBC) due to change in the discount rate	$ 589
Pension expense	7,600	$ 7,000	$ 6,800
Regulatory Assets	$ 140,300	149,100
Defined Benefit Plan, Expected Long-term Rate-of-Return on Assets Assumption	The desired investment objective is a long-term rate of return on assets that is approximately 5 – 6% greater than the assumed rate of inflation as measured by the Consumer Price Index.
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation	$ 142,800	150,600
Company's contributions	16,628	4,100	5,146
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Company's contributions	4,000	4,000	4,000
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation	9,500	9,500
Company's contributions	401	34	34
Fair Value Of Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Pension expense	7,200	7,600	7,700
Defined Benefit Obligations
Defined Benefit Plan Disclosure [Line Items]
Regulatory Assets	72,000	84,500
Four Zero One K Plan
Defined Benefit Plan Disclosure [Line Items]
Company's contributions	$ 2,700	$ 2,400	$ 2,300
Benefit Plan Costs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Long-term Return on Assets	7.75%	7.75%	8.00%
Scenario Forecast \| Pension Plans \| Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations				53.00%
Scenario Forecast \| Pension Plans \| Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations				37.00%
Scenario Forecast \| Other Postretirement Benefit Plans, Defined Benefit \| Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations				55.00%
Scenario Forecast \| Other Postretirement Benefit Plans, Defined Benefit \| Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations				45.00%
Real Estate Funds \| Scenario Forecast \| Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations				10.00%