Summary of Significant Accounting Policies - Premises and Equipment (Details)	12 Months Ended
	Dec. 31, 2017 building
Premises and Equipment
Number of buildings owned	1
Building
Premises and Equipment
Estimated useful life	40 years
Furniture | Minimum
Premises and Equipment
Estimated useful life	5 years
Furniture | Maximum
Premises and Equipment
Estimated useful life	15 years
Equipment | Minimum
Premises and Equipment
Estimated useful life	5 years
Equipment | Maximum
Premises and Equipment
Estimated useful life	15 years
Leasehold improvements | Minimum
Premises and Equipment
Estimated useful life	5 years
Leasehold improvements | Maximum
Premises and Equipment
Estimated useful life	15 years