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Derivatives - Derivatives Associated With Cash Flow Hedges (Details) - Cash Flow Hedge - Hedging Instruments And Hedged Items - USD ($)
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]        
Notional     $ 650,000,000  
Variability in cash flows related to debt instruments (primarily loans)       $ 250,000,000
Interest Rate Contract        
Derivative Instruments, Gain (Loss) [Line Items]        
Variability in cash flows related to debt instruments (primarily loans) $ 900,000,000 $ 900,000,000    
Interest Rate Swap        
Derivative Instruments, Gain (Loss) [Line Items]        
Notional 900,000,000 900,000,000    
Cash Flow Hedge Derivative Instrument Assets at Fair Value   888,000    
Liabilities $ 891,000 $ 5,000