Retirement Benefit Plans - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018
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		$ 540
Pension expense		7,000	$ 6,800	$ 7,300
Regulatory Assets		$ 149,100	142,000
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]
Amortizations related to Actuarial Loss and Prior Service Cost		$ 6,100
Accumulated Benefit Obligation		150,600	135,200
Company's contributions		4,100	5,146	4,215
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Amortizations related to Actuarial Loss and Prior Service Cost		2,700
Company's contributions		4,000	4,000	4,000
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Amortizations related to Actuarial Loss and Prior Service Cost		700
Accumulated Benefit Obligation		9,500	6,900
Company's contributions		34	34	40
Fair Value Of Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Pension expense		7,600	7,700	7,300
Defined Benefit Obligations
Defined Benefit Plan Disclosure [Line Items]
Regulatory Assets		84,500	75,900
Four Zero One K Plan
Defined Benefit Plan Disclosure [Line Items]
Company's contributions		$ 2,434	$ 2,304	$ 2,098
Benefit Plan Costs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Long-term Return on Assets		7.75%	8.00%	8.00%
Scenario Forecast | Pension Plans | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations					47.00%
Scenario Forecast | Pension Plans | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations					37.00%
Scenario Forecast | Pension Plans | Equity And Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations	[1]				6.00%
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%

[1] Represents investments in an asset allocation fund. This fund invests in both equity and debt securities.