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Investment Securities Available for Sale - Narrative (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]      
Proceeds from sales of investment securities $ 10,100,000 $ 37,300,000  
Realized net losses on sales of investment securities available for sale $ 3,400,000 1,700,000  
Contractual maturities (up to) 30 years    
Weighted average life of available for sale securities portfolio 5 years 299 days   7 years 47 days
Other-than-temporary impairment recorded in earnings $ 0 $ 0