Summary of Significant Accounting Policies - Office Properties and Equipment (Details)	12 Months Ended
	Dec. 31, 2017
Buildings and related components | Minimum
Office Properties and Equipment
Useful life	10 years
Buildings and related components | 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