Commitments and Contingent Liabilities - Commitments to Extend Credit, Participations Sold (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Commitments to extend credit
Commitments
Participations sold to other institutions	$ 43.8	$ 49.1
Standby letters of credit
Commitments
Participations sold to other institutions	$ 17.8	$ 17.8

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