Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Rights - Initial Recognition (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) item	Sep. 30, 2019 USD ($) item	Sep. 30, 2020 USD ($) item	Sep. 30, 2019 USD ($) item	Dec. 31, 2019 USD ($) item
Mortgage servicing rights
Inputs
Amount recognized | $	$ 245,946,000	$ 246,757,000	$ 753,795,000	$ 545,839,000
Fair Values | Pricing spread | Level 3 | Minimum
Inputs:
Input				5.5
Fair Values | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input				7.7
Fair Values | Mortgage servicing rights | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $			78		$ 77
Fair Values | Mortgage servicing rights | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $			110		100
Fair Values | Mortgage servicing rights | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $			$ 104		$ 97
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Minimum
Inputs:
Input	8.0		8.0		6.8
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Maximum
Inputs:
Input					15.8
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Weighted average
Inputs:
Input	10.1		10.1		8.5
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	10.4		10.4		9.3
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	32.3		32.3		40.9
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input				15.0
Input	15.3		15.3		12.7
Fair Values | Mortgage servicing rights | Life | Level 3 | Minimum
Inputs:
Input	1.7		1.7		1.4
Fair Values | Mortgage servicing rights | Life | Level 3 | Maximum
Inputs:
Input	6.8		6.8		7.4
Fair Values | Mortgage servicing rights | Life | Level 3 | Weighted average
Inputs:
Input	5.4		5.4		6.1
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
Inputs
Amount recognized | $	$ 245,946,000	246,757,000	$ 753,795,000	$ 545,839,000
Unpaid principal balance of underlying loans | $	$ 25,369,941,000	$ 15,709,249,000	$ 63,766,627,000	$ 35,532,425,000
Weighted-average servicing fee rate (as a percent)	0.32%	0.43%	0.36%	0.42%
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $	$ 80	$ 78	$ 77	$ 78
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $	110	100	110	100
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $	$ 102	$ 97	$ 100	$ 97
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Minimum
Inputs:
Input	8.0	5.5	6.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input	17.6	16.2	18.1	16.2
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Weighted average
Inputs:
Input	9.6	8.3	9.3	8.6
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	7.2	8.8	7.2
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	41.0	32.1	49.8	32.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	10.4	15.7	12.4
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input	2.3	2.7	1.5	2.6
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input	9.1	7.5	9.1	7.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input	7.3	5.5	6.6	5.8