Retirement Benefit Plans - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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			$ 524
Pension expense			6,800	$ 7,300	$ 4,900
Regulatory Assets			$ 142,000	126,400
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			$ 5,000
Accumulated Benefit Obligation			135,200	126,800
Company's contributions			5,146	4,215	4,191
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Amortizations related to Actuarial Loss and Prior Service Cost			3,500
Company's contributions			4,000	4,000	3,650
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Amortizations related to Actuarial Loss and Prior Service Cost			500
Accumulated Benefit Obligation			6,900	7,000
Company's contributions			34	40	53
Fair Value Of Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Pension expense			7,700	7,300	4,300
Defined Benefit Obligations
Defined Benefit Plan Disclosure [Line Items]
Regulatory Assets			75,900	64,700
Four Zero One K Plan
Defined Benefit Plan Disclosure [Line Items]
Company's contributions			$ 2,304	$ 2,098	$ 1,877
Benefit Plan Costs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Long-term Return on Assets			8.00%	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.