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Summary of Significant Accounting Policies - Premises and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Transfers and Servicing of Financial Assets
Mortgage loans serviced for others	$ 711.1	$ 695.1
Buildings | Minimum
Premises and equipment
Useful lives	25 years
Buildings | Maximum
Premises and equipment
Useful lives	40 years
Building improvements | Minimum
Premises and equipment
Useful lives	3 years
Building improvements | Maximum
Premises and equipment
Useful lives	15 years
Furniture and equipment | Minimum
Premises and equipment
Useful lives	3 years
Furniture and equipment | Maximum
Premises and equipment
Useful lives	10 years

X

- Definition

Fair value as of the balance sheet date of mortgage loans representing net future revenues from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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