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Retirement Benefit Plans - Additional Information (Detail) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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			$ 472,000
Pension expense			7,300,000	$ 4,900,000	$ 6,600,000
Regulatory Assets			$ 126,400,000	145,400,000
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			$ 4,700,000
Accumulated Benefit Obligation			126,800,000	121,800,000
Company's contributions			4,215,000	4,191,000	3,700,000
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Amortizations related to Actuarial Loss and Prior Service Cost			2,500,000
Company's contributions			4,000,000	3,650,000	3,280,000
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Amortizations related to Actuarial Loss and Prior Service Cost			300,000
Accumulated Benefit Obligation			7,000,000	6,300,000
Company's contributions			40,000	53,000	53,000
Fair Value Of Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Pension expense			7,300,000	4,300,000	6,600,000
Defined Benefit Obligations
Defined Benefit Plan Disclosure [Line Items]
Regulatory Assets			64,700,000	65,100,000
Four Zero One K Plan
Defined Benefit Plan Disclosure [Line Items]
Company's contributions			$ 2,098,000	$ 1,877,000	$ 1,678,000
Benefit Plan Costs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Long-term Return on Assets			8.00%	8.00%	8.50%
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.

X

- Definition

For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Accumulated Benefit Obligation
-URI http://asc.fasb.org/extlink&oid=6503844

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Expected Long-Term Rate of Return on Plan Assets
-URI http://asc.fasb.org/extlink&oid=6512046

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The increase in the fair value of plan assets from contributions made by the employer.

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The amount of increase or decrease in net periodic benefit cost related to the effects of a change in the substantive terms of an existing defined benefit plan or the initiation of a new plan. A plan amendment may increase or decrease benefits, including those attributed to years of service already rendered.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Amendment
-URI http://asc.fasb.org/extlink&oid=49173939

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- Definition

A narrative description of the basis used to determine the overall expected long-term rate-of-return on assets assumption, such as the general approach used, the extent to which the overall rate-of-return on assets assumption was based on historical returns, the extent to which adjustments were made to those historical returns to reflect expectations of future returns, and how those adjustments were determined. The description would generally consider the major categories of assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

Target allocation of investments of this type to total plan assets presented on a weighted average basis.

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Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position.

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The amount of pension benefit costs recognized during the period for (1) defined benefit plans and (2) defined contribution plans. For defined benefit plans, pension expense includes the following components: service cost, interest cost, expected return on plan assets, gain (loss) on plan assets, prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. For defined contribution plans, the pension expense generally equals the firm's contribution to employees' accounts (if the firm contributes) during the period.

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Change in Discount Rate Assumption

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