Other Assets - Quantitative Assumptions Used for MSRs (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Minimum
Quantitative assumptions used in determining lower of cost or fair value of MSRs
Conditional prepayment rate (as a percent)	8.53%	8.61%
Life (of the MSR)	3 years 3 months 15 days	3 years 9 months
Weighted-average coupon rate (as a percent)	3.97%	3.99%
Discount rate (as a percent)	10.50%	10.46%
Maximum
Quantitative assumptions used in determining lower of cost or fair value of MSRs
Conditional prepayment rate (as a percent)	19.63%	18.01%
Life (of the MSR)	7 years 1 month 24 days	7 years 3 months 4 days
Weighted-average coupon rate (as a percent)	6.79%	6.87%
Discount rate (as a percent)	10.52%	10.52%
Weighted Average
Quantitative assumptions used in determining lower of cost or fair value of MSRs
Conditional prepayment rate (as a percent)	9.04%	9.16%
Life (of the MSR)	6 years 9 months 4 days	6 years 2 months 9 days
Weighted-average coupon rate (as a percent)	4.04%	4.06%
Discount rate (as a percent)	10.50%	10.50%