Contingencies and Other Disclosures (Details) shares in Millions	9 Months Ended	36 Months Ended	48 Months Ended
	Sep. 30, 2018 USD ($) entity shares	Dec. 31, 2007 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Estimated litigation liability	$ 32,800,000
Number of GSEs to which conventional conforming single-family mortgage loans were predominately sold to | entity	2
Loan-to-value ratio at origination	80.00%
Subservicing agreement, period	3 years
Mortgage loans originated and sold to agencies			$ 69,500,000,000
Loans sold to GSEs			57,600,000,000
Loans guaranteed by Ginnie Mae			$ 11,900,000,000
Loans included in FH proprietary securitizations		$ 26,700,000,000
Accrued losses on loan repurchase exposure	$ 32,500,000			$ 34,200,000
Subservicer expenditure reimbursement amount disputed
Loss Contingencies [Line Items]
Actual damages sought by plaintiff	$ 43,500,000
Visa Class B Shares
Loss Contingencies [Line Items]
Number of Visa Class B shares (in shares) | shares	2.4
Mortgage Securitization Litigation
Loss Contingencies [Line Items]
Investment in proprietary securitizations subject to lawsuits	$ 83,400,000
Investment in proprietary securitizations subject to indemnifications	231,200,000
Minimum
Loss Contingencies [Line Items]
Estimated reasonably possible losses in excess of currently established liabilities	0
Maximum
Loss Contingencies [Line Items]
Estimated reasonably possible losses in excess of currently established liabilities	$ 20,000,000