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Derivatives and Risk Management Activities - Interest Rate Risk Hedging (Details) - Cash flow hedge
9 Months Ended
Sep. 30, 2018
USD ($)
contract
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 | $ $ 200,000,000
Average rate locked (percent) 2.83%
8 forward starting interest rate swaps (30-year), 3.06%  
Interest Rate Risk Hedging  
Number of interest rate derivatives | contract 8
Term of derivative contract 30 years
Notional amount of derivatives | $ $ 200,000,000
Average rate locked (percent) 3.06%