Fair Value of Financial Instruments - Additional Information (Details)	Jun. 30, 2018	Dec. 31, 2017
Effective rate - minimum [Member] | Taxable Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Effective rate	8.10%	7.90%
Effective rate - minimum [Member] | MRBs and Bond Purchase Commitments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Effective rate	3.20%	2.90%
Effective rate - maximum [Member] | Taxable Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Effective rate	9.30%	9.20%
Effective rate - maximum [Member] | MRBs and Bond Purchase Commitments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Effective rate	8.50%	8.80%