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Summary of significant accounting policies - Estimated Useful Lives of Related Assets (Details)
12 Months Ended
Dec. 31, 2018
Minimum | Premises and leasehold improvements  
Property, Plant and Equipment  
Property, plant and equipment, estimated useful life 7 years
Minimum | Furniture, fixtures and equipment  
Property, Plant and Equipment  
Property, plant and equipment, estimated useful life 3 years
Maximum | Premises and leasehold improvements  
Property, Plant and Equipment  
Property, plant and equipment, estimated useful life 40 years
Maximum | Furniture, fixtures and equipment  
Property, Plant and Equipment  
Property, plant and equipment, estimated useful life 10 years