Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Rights - Initial Recognition (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) item	Mar. 31, 2019 USD ($) item	Dec. 31, 2019 USD ($) item
Mortgage servicing rights
Inputs
Amount recognized | $	$ 282,315,000	$ 115,751,000
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 | $	112		100
Fair Values | Mortgage servicing rights | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $	$ 108		$ 97
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Minimum
Inputs:
Input	8.3		6.8
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Maximum
Inputs:
Input	18.1		15.8
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Weighted average
Inputs:
Input	10.7		8.5
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	9.7		9.3
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	27.9		40.9
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	16.5		12.7
Fair Values | Mortgage servicing rights | Life | Level 3 | Minimum
Inputs:
Input	1.3		1.4
Fair Values | Mortgage servicing rights | Life | Level 3 | Maximum
Inputs:
Input	7.2		7.4
Fair Values | Mortgage servicing rights | Life | Level 3 | Weighted average
Inputs:
Input	5.0		6.1
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
Inputs
Amount recognized | $	$ 282,315,000	115,751,000
Unpaid principal balance of underlying loans | $	$ 18,330,384,000	$ 8,145,850,000
Weighted-average servicing fee rate (as a percent)	0.40%	0.39%
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $	$ 77	$ 78
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $	100	100
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $	$ 97	$ 95
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Minimum
Inputs:
Input	6.8	5.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input	15.6	15.6
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Weighted average
Inputs:
Input	8.2	8.9
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	9.1	5.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	49.8	73.0
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	14.5	15.3
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input	1.5	0.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input	7.8	10.2
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input	5.9	5.8