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Benefit Plans Components of Benefit Plans (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Number_of_defined_benefit_plan_investment_funds
Sep. 30, 2013
Number_of_qualified_defined_benefit_pension_plans
Number_of_defined_benefit_plan_structures
plans
Number_of_qualified_defined_contribution_plans
Years
Dec. 31, 2012
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2011
Dec. 31, 2010
Defined Benefit Plan Disclosure                
Number of defined benefit plans     1          
Number of definied contribution plans     1          
Number of unfunded post-retirement health care plans     2          
Number of benefit plan structures     2          
Number of consecutive highest base pay years     3          
Percentage of straight-time earnings for cash balance benefit credit     6.00%          
Percentage added to consumer price index for interest credit of cash balance benefit     3.00%          
Minimum interest credit percentage for cash balance benefit     6.00%          
Maximum interest credit percentage for cash balance benefit     10.00%          
Rate of cash balance benefit credit for calendar year   6.00%     6.00%      
Number of defined benefit plan investment funds   2            
Fixed and variable fund annual maximum contribution         $ 10,000      
Fixed fund balance interest credits after January 1, 2010     6.00%          
Fixed fund balance credit percentage subtracted from the actuarial rate of return after January 1, 2010     0.50%          
Fixed fund interest credit rate     6.00% 6.00%        
Fixed fund interest credit amount     36,000,000   38,000,000      
Defined contribution plan contribution amount     34,000,000   34,000,000   31,000,000  
Market-related value phase-in period number of years     3          
Number of years during which cost of living adjustment changed 4 years              
Cost of Living Adjustment Cap   2.30%   0.00%   1.15%   0.00%
Cost of living adjustment cap for calendar year 2014 and forward 5.00%              
Retirement age of eligibility for cost of living adjustment after January 1, 2010               60
Original benefit structure
                
Defined Benefit Plan Disclosure                
Defined contribution plan employer matching contribution rate     0.25          
Defined contribution plan, maximum annual contribution per employee, percent     1.50%          
Cash Balance Benefit Structure
                
Defined Benefit Plan Disclosure                
Defined contribution plan employer matching contribution rate     $ 0.75          
Defined contribution plan, maximum annual contribution per employee, percent     4.50%