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Summary of significant accounting policies - Estimated Useful Lives of Related Assets (Details)
12 Months Ended
Dec. 31, 2019
Premises and leasehold improvements | Minimum  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment, estimated useful life 7 years
Premises and leasehold improvements | Maximum  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment, estimated useful life 40 years
Furniture, fixtures and equipment | Minimum  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment, estimated useful life 3 years
Furniture, fixtures and equipment | Maximum  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment, estimated useful life 10 years