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Summary of Significant Accounting Policies - Office Properties and Equipment (Details)
 12 Months Ended
Dec. 31, 2019
Office buildings and improvements | Minimum  
Office Properties and Equipment  
Useful life 10 years
Office buildings and improvements | Maximum  
Office Properties and Equipment  
Useful life 40 years
Furniture, fixtures and equipment | Minimum  
Office Properties and Equipment  
Useful life 3 years
Furniture, fixtures and equipment | Maximum  
Office Properties and Equipment  
Useful life 10 years