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Securities - Investment Portfolio (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Securities      
Pretax realized losses on sale of available-for-sale securities $ 1,893,000.000 $ 3,888,000.000 $ 284,000.000
FHLB and FRB Stock      
FHLBC stock 5,400,000 $ 3,100,000  
FRB stock $ 4,800,000