Fair Value - Significant Unobservable Inputs Used In the Valuation of MSRs (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Total amount of loans being serviced for the benefit of others at the end of the period	$ 634.5	$ 627.3
Minimum
Assumption for Fair Value as of Balance Sheet Date of Assets or Liabilities that relate to Transferor's Continuing Involvement [Line Items]
Prepayment speed	6.90%	6.50%
Maturity (months)	19 months	20 months
Discount rate	9.50%	9.50%
Maximum
Assumption for Fair Value as of Balance Sheet Date of Assets or Liabilities that relate to Transferor's Continuing Involvement [Line Items]
Prepayment speed	14.30%	13.50%
Maturity (months)	97 months	104 months
Discount rate	12.00%	12.00%