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Derivatives - Derivatives Associated With Cash Flow Hedges (Details) - Cash Flow Hedges - Hedging Instruments And Hedged Items - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Mar. 31, 2017
Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]        
Notional     $ 650,000  
Variability in cash flows related to debt instruments (primarily loans)       $ 250,000
Interest Rate Contract        
Derivative Instruments, Gain (Loss) [Line Items]        
Variability in cash flows related to debt instruments (primarily loans) $ 900,000 $ 900,000    
Interest rate swaps        
Derivative Instruments, Gain (Loss) [Line Items]        
Notional 900,000 900,000    
Assets $ 373 $ 942