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Fair Value Measurements - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Medical benefit component for 401h accounts (in dollars) $ 9 $ 9
Weather derivative premium (in dollars) $ 10 $ 3
Equity Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percent of total assets (as a percent) 72.00% 70.00%
Equity Securities [Member] | Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Target plan asset allocations (as a percent) 70.00% 70.00%
Equity Securities [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Target plan asset allocations (as a percent) 95.00% 95.00%
Fixed Income Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percent of total assets (as a percent) 28.00% 29.00%
Fixed Income Securities [Member] | Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Target plan asset allocations (as a percent) 5.00% 5.00%
Fixed Income Securities [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Target plan asset allocations (as a percent) 20.00% 20.00%
Cash [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percent of total assets (as a percent)   1.00%
Cash [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Target plan asset allocations (as a percent) 10.00% 10.00%