Summary of Significant Accounting Policies - Office properties and equipment (Details)	12 Months Ended
	Dec. 31, 2015
Office buildings and improvements | Minimum
Office properties and equipment
Useful lives	10 years
Office buildings and improvements | Maximum
Office properties and equipment
Useful lives	40 years
Furniture, fixtures and equipment | Minimum
Office properties and equipment
Useful lives	3 years
Furniture, fixtures and equipment | Maximum
Office properties and equipment
Useful lives	10 years