Fair Value - Schedule of Carrying Amounts and Estimated Fair Values of Financial Instruments and Nonfinancial Assets Measured at Fair Value on a Recurring or Non-recurring basis or Disclosed, but not Carried, at Fair Value (Footnote) (Details) - Impaired Government Insured Stratum [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset at carrying value	$ 158.0	$ 172.9
Valuation allowance of MSRs	$ 24.8	$ 28.2

Fair Value - Schedule of Carrying Amounts and Estimated Fair Values of Financial Instruments and Nonfinancial Assets Measured at Fair Value on a Recurring or Non-recurring basis or Disclosed, but not Carried, at Fair Value (Footnote) (Details) - Impaired Government Insured Stratum [Member] - USD ($)
$ in Millions

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Servicing asset at carrying value

$ 158.0

$ 172.9

Valuation allowance of MSRs

$ 24.8

$ 28.2