Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Rights - Initial Recognition (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) item	Jun. 30, 2019 USD ($) item	Jun. 30, 2020 USD ($) item	Jun. 30, 2019 USD ($) item	Dec. 31, 2019 USD ($) item
Mortgage servicing rights
Inputs
Amount recognized | $	$ 225,534,000	$ 183,331,000	$ 507,849,000	$ 299,082,000
Fair Values | Pricing spread | Level 3 | Minimum
Inputs:
Input				5.8
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 | $			79		$ 77
Fair Values | Mortgage servicing rights | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $			111		100
Fair Values | Mortgage servicing rights | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $			$ 106		$ 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	10.2		10.2		8.5
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	10.5		10.5		9.3
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	33.4		33.4		40.9
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input				14.4
Input	16.9		16.9		12.7
Fair Values | Mortgage servicing rights | Life | Level 3 | Minimum
Inputs:
Input	1.3		1.3		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	5.0		5.0		6.1
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
Inputs
Amount recognized | $	$ 225,534,000	183,331,000	$ 507,849,000	$ 299,082,000
Unpaid principal balance of underlying loans | $	$ 20,066,302,000	$ 11,677,325,000	$ 38,396,686,000	$ 19,823,176,000
Weighted-average servicing fee rate (as a percent)	0.36%	0.42%	0.38%	0.41%
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $	$ 79	$ 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 | $	$ 101	$ 97	$ 99	$ 96
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Minimum
Inputs:
Input	8.1	5.9	6.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input	18.1	15.8	18.1	15.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Weighted average
Inputs:
Input	9.9	8.7	9.1	8.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	8.8	8.7	8.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	29.1	32.8	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	13.0	13.8	13.7
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input	2.9	2.6	1.5	2.6
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input	8.1	7.8	8.1	7.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input	6.3	6.1	6.1	6.0