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Hedges and Derivative Financial Instruments - Narrative (Details)
3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
contract
Dec. 31, 2017
USD ($)
contract
General Discussion of Derivative Instruments and Hedging Activities [Abstract]    
Number of interest rate derivatives held (in contracts) | contract 0 0
Net hedge ineffectiveness gain (loss) $ 0 $ 0
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months $ 44,000,000