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Retirement Benefit Plans - Additional Information (Detail) (USD $)
 1 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended 12 Months Ended
Jun. 25, 2010
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Aug. 17, 2006
Pension Protection Act
Jul. 06, 2012
Well-Funded Pension Plan
Dec. 31, 2012
Well-Funded Pension Plan
Dec. 31, 2011
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, 2012
Defined Benefit Obligations
Dec. 31, 2011
Defined Benefit Obligations
Dec. 31, 2012
Fair Value Of Plan Assets
Dec. 31, 2011
Fair Value Of Plan Assets
Dec. 31, 2010
Fair Value Of Plan Assets
Dec. 31, 2012
Total contributions
Well-Funded Pension Plan
Dec. 31, 2012
Pension Plans
Dec. 31, 2011
Pension Plans
Dec. 31, 2010
Pension Plans
Dec. 31, 2012
Other Postretirement Benefit Plans, Defined Benefit
Dec. 31, 2010
Other Postretirement Benefit Plans, Defined Benefit
Dec. 31, 2012
Other Postretirement Benefit Plans, Defined Benefit
Equity Funds
Dec. 31, 2012
Other Postretirement Benefit Plans, Defined Benefit
Fixed Income Securities
Dec. 31, 2012
Supplemental Employee Retirement Plans, Defined Benefit
Dec. 31, 2011
Supplemental Employee Retirement Plans, Defined Benefit
Dec. 31, 2010
Supplemental Employee Retirement Plans, Defined Benefit
Dec. 31, 2012
Pension Plans
Dec. 31, 2011
Pension Plans
Dec. 31, 2010
Pension Plans
Dec. 31, 2012
Pension Plans
Equity Funds
Dec. 31, 2012
Pension Plans
Fixed Income Securities
Dec. 31, 2012
Pension Plans
Equity And Debt Securities
Dec. 31, 2012
Four Zero One K Plan
Dec. 31, 2011
Four Zero One K Plan
Dec. 31, 2010
Four Zero One K Plan
Dec. 31, 2012
Scenario, Forecast
Pension Plans
Dec. 31, 2012
Scenario, Forecast
Other Postretirement Benefit Plans, Defined Benefit
Dec. 31, 2012
Scenario, Forecast
Supplemental Employee Retirement Plans, Defined Benefit
Dec. 31, 2013
Scenario, Forecast
Pension Plans
Equity Funds
Dec. 31, 2013
Scenario, Forecast
Pension Plans
Fixed Income Securities
Dec. 31, 2013
Scenario, Forecast
Pension Plans
Equity And Debt Securities
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   $ 367,000 $ 325,000 $ 300,000                                                                                
Increase in Rate of Compensation   3.00% 3.50% 3.50%                                                                                
Amortizations related to Actuarial Loss and Prior Service Cost                                                                             4,400,000 2,500,000 200,000      
Pension expense   6,300,000 6,200,000 5,600,000                       6,700,000 5,700,000 6,200,000                                                    
Regulatory Assets   182,600,000 185,800,000                     62,500,000 55,300,000                                                          
Accumulated Benefit Obligation                                       103,400,000 91,300,000           4,800,000 500,000                                
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             (3,617,000) (3,132,000) (2,406,000) (129,000)       (64,000) (78,000) (133,000)                              
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 1,100,000 8,300,000                     9,400,000       2,190,000 3,482,000     53,000 53,000 53,000 9,387,000 8,813,000 4,302,000       1,387,000 1,190,000 980,000            
Minimum required contribution                 $ 1,000,000   $ 6,100,000                                                                  
Defined Benefit Plan, Target Plan Asset Allocations                                                 55.00% 45.00%             48.00% 47.00% 5.00%             48.00% 47.00% 5.00% [1]
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.                                                                                    
[1] Represents investments in an asset allocation fund. This fund invests in both equity and debt securities.