Segments - Additional Information (Details)	9 Months Ended
	Sep. 30, 2018 Property Unit Security Segment Rating
Segment Reporting Information [Line Items]
Number of Reportable Segments | Segment	4
Mortgage Revenue Bond Investments Segment [Member]
Segment Reporting Information [Line Items]
Number of Available for Sale Securities | Security	78
Real Estate [Member]
Segment Reporting Information [Line Items]
Number of Real Estate Properties | Property	2
Number of rental units under MF properties segment. | Unit	859
Residential Mortgage | Mortgage Revenue Bond Investments Segment [Member]
Segment Reporting Information [Line Items]
Number of rental units financed by mortgage revenue bonds | Unit	10,746
Commercial Real Estate [Member] | Mortgage Revenue Bond Investments Segment [Member]
Segment Reporting Information [Line Items]
Number of collateralized securities | Security	1
Minimum [Member]
Segment Reporting Information [Line Items]
Required rating for tax exempted investments other than mortgage revenue bonds | Rating	1
Maximum [Member]
Segment Reporting Information [Line Items]
Required rating for tax exempted investments other than mortgage revenue bonds | Rating	4
TaxExempt Securities Other Than MRBs And Other Investments [Member]
Segment Reporting Information [Line Items]
Assets percentage	25.00%