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Securities Securities - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]          
Securities pledged as collateral to secure public funds and other deposits or securities sold under agreements to repurchase $ 1,500,000,000   $ 1,500,000,000    
Secured borrowing under agreement to repurchase 128,739,000   128,739,000    
Accumulated unrealized losses 56,955,000   56,955,000   $ 17,547,000
Fair value of securities 2,321,665,000   2,321,665,000   $ 1,379,070,000
Debt Securities Available-for-sale Sold Amount     14,500,000    
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax 0 $ 0 22,000 $ 0  
Securities pledged as collateral          
Debt Securities, Available-for-sale [Line Items]          
Secured borrowing under agreement to repurchase $ 128,739,000   $ 128,739,000