Office Properties and Equipment, net - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2017
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 863	$ 4,900	$ 3,800	$ 3,400
Land and building improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, additions		1,300	$ 8,900
Stewardship Property
Property, Plant and Equipment [Line Items]
Assets held-for-sale, long-lived, fair value disclosure		$ 1,800