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Derivative Instruments and Hedging Activities - Interest Rate Swaps and Interest Rate Caps (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative [Line Items]      
Interest income-investment securities $ 43,909 $ 33,696 $ 36,320
Interest expense-investment securities 25,344 17,764 13,737
Designated as Hedging Instrument | Interest rate swaps      
Derivative [Line Items]      
Interest income-investment securities 267 0 0
Interest expense-investment securities $ 0 $ 743 $ 1,619