Mortgage Servicing Rights - Quantitative Assumptions Used for MSRs (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Minimum
Quantitative assumptions used in determining lower of cost or fair value of MSRs
Conditional prepayment rate (as a percent)	7.76%	8.53%
Life (of the MSR)	3 years 4 months 24 days	3 years 3 months 15 days
Weighted-average coupon rate (as a percent)	3.96%	3.97%
Discount rate (as a percent)	10.50%	10.50%
Maximum
Quantitative assumptions used in determining lower of cost or fair value of MSRs
Conditional prepayment rate (as a percent)	18.94%	19.63%
Life (of the MSR)	8 years 4 days	7 years 1 month 24 days
Weighted-average coupon rate (as a percent)	6.77%	6.79%
Discount rate (as a percent)	10.50%	10.52%
Weighted Average
Quantitative assumptions used in determining lower of cost or fair value of MSRs
Conditional prepayment rate (as a percent)	8.12%	9.04%
Life (of the MSR)	7 years 4 months 6 days	6 years 9 months 4 days
Weighted-average coupon rate (as a percent)	4.02%	4.04%
Discount rate (as a percent)	10.50%	10.50%