Significant Accounting Policies and Recently Issued Accounting Pronouncement - Mortgage Servicing Rights and Mortgage Servicing Liabilities (Details)	12 Months Ended
	Dec. 31, 2018 item
Mortgage servicing rights
Mortgage Servicing Rights
Number of classes of MSRs	3
Mortgage servicing rights | Fixed Rate Stratified Mortgage Pool
Mortgage Servicing Rights
Stratification range, as a percent	0.50%
Mortgage servicing rights | Fixed Rate Stratified Mortgage Pool | Minimum
Mortgage Servicing Rights
Note rates (as a percent)	3.00%
Mortgage servicing rights | Fixed Rate Stratified Mortgage Pool | Maximum
Mortgage Servicing Rights
Note rates (as a percent)	4.50%
Mortgage servicing rights | Fixed Rate Single Mortgage Pool | Maximum
Mortgage Servicing Rights
Note rates (as a percent)	3.00%
Mortgage Servicing Rights Class One [Member]
Mortgage Servicing Rights
Interest rate (as a percent)	4.50%
Mortgage Servicing Rights Class Two [Member]
Mortgage Servicing Rights
Interest rate (as a percent)	4.50%