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.

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=28361610&loc=d3e1928-114920

Retirement Benefit Plans - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended			1 Months Ended				1 Months Ended		1 Months Ended			12 Months Ended
	Jun. 25, 2010	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Aug. 17, 2006 Pension Protection Act	Jul. 06, 2012 Well-Funded Pension Plan	Jul. 06, 2012 Well-Funded Pension Plan Minimum	Aug. 17, 2006 Well-Funded Pension Plan Minimum	Jul. 06, 2012 Well-Funded Pension Plan Maximum	Aug. 17, 2006 Well-Funded Pension Plan Maximum	Jan. 31, 2010 Employee Retirement Income Security Act Class Action	Dec. 31, 2013 Defined Benefit Obligations	Dec. 31, 2012 Defined Benefit Obligations	Dec. 31, 2013 Fair Value Of Plan Assets	Dec. 31, 2012 Fair Value Of Plan Assets	Dec. 31, 2011 Fair Value Of Plan Assets	Dec. 31, 2013 Pension Plans	Dec. 31, 2012 Pension Plans	Dec. 31, 2011 Pension Plans	Dec. 31, 2013 Pension Plans Equity Funds	Dec. 31, 2013 Pension Plans Fixed Income Securities	Dec. 31, 2013 Pension Plans Real Estate Funds	Dec. 31, 2013 Pension Plans Equity And Debt Securities	Dec. 31, 2013 Other Postretirement Benefit Plans, Defined Benefit	Dec. 31, 2012 Other Postretirement Benefit Plans, Defined Benefit	Dec. 31, 2013 Other Postretirement Benefit Plans, Defined Benefit Equity Funds	Dec. 31, 2013 Other Postretirement Benefit Plans, Defined Benefit Fixed Income Securities	Dec. 31, 2013 Pension Plans	Dec. 31, 2012 Pension Plans	Dec. 31, 2011 Pension Plans	Dec. 31, 2013 Supplemental Employee Retirement Plans, Defined Benefit	Dec. 31, 2012 Supplemental Employee Retirement Plans, Defined Benefit	Dec. 31, 2011 Supplemental Employee Retirement Plans, Defined Benefit	Dec. 31, 2013 Four Zero One K Plan	Dec. 31, 2012 Four Zero One K Plan	Dec. 31, 2011 Four Zero One K Plan	Dec. 31, 2013 Scenario, Forecast Other Postretirement Benefit Plans, Defined Benefit	Dec. 31, 2013 Scenario, Forecast Pension Plans	Dec. 31, 2013 Scenario, Forecast Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Change in Discount Rate		0.25%	0.25%	0.25%
Increase or decrease of Net Periodic Benefit Cost (NPBC) due to change in the discount rate		$ 431,000	$ 367,000	$ 325,000
Increase in Rate of Compensation		3.00%	3.00%	3.50%
Amortizations related to Actuarial Loss and Prior Service Cost																																					1,700,000	3,100,000	100,000
Pension expense		6,600,000	6,300,000	6,200,000										6,600,000	6,700,000	5,700,000
Regulatory Assets		143,700,000	186,500,000									42,600,000	62,500,000
Accumulated Benefit Obligation																												96,900,000	103,400,000		5,100,000	4,800,000
Defined Benefit Plan, Funded Percentage					100.00%	100.00%		92.00%		100.00%	80.00%
Amortization Period For Defined Benefit Plan					7 years						7 years
Defined Benefit Plan, shortfall											10,200,000													(786,000)	(129,000)			(4,229,000)	(3,617,000)	(3,132,000)	(184,000)	(64,000)	(78,000)
Defined Benefit Plan, Annual Employer Contributions											1,700,000
Defined Benefit Plan, Description of Settlements and Curtailments	The Company evaluated the two alternative shortfall amortization periods under the Relief Act and made the decision to continue with the seven year amortization period.
Company's contributions						3,100,000											3,700,000	9,387,000	8,813,000					3,280,000	2,190,000						53,000	53,000	53,000	1,678,000	1,387,000	1,190,000
Minimum required contribution							$ 1,000,000		$ 6,100,000
Defined Benefit Plan, Target Plan Asset Allocations																				48.00%	37.00%	10.00%	5.00%			55.00%	45.00%
Defined Benefit Plan, Expected Long-term Return on Assets		8.50%
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.