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Derivatives and Risk Management Activities - Interest Rate Risk Hedging (Details) - Cash flow hedge
$ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
contract
16 forward starting interest rate swaps (30-year), 2.86%  
Interest Rate Risk Hedging  
Number of interest rate derivatives | contract 16
Term of derivative contract 30 years
Notional amount of derivatives | $ $ 400
Average rate locked (percent) 2.86%
8 forward starting interest rate swaps (30-year), 2.83%  
Interest Rate Risk Hedging  
Number of interest rate derivatives | contract 8
Term of derivative contract 30 years
Notional amount of derivatives | $ $ 200
Average rate locked (percent) 2.83%