Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Rights - Initial Recognition (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($) item	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($) item	Dec. 31, 2018 USD ($) item
Mortgage servicing rights
Inputs
Amount recognized | $	$ 246,757,000	$ 149,000,000	$ 545,839,000	$ 448,604,000
Fair Values | Mortgage servicing rights | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $			77		$ 78
Fair Values | Mortgage servicing rights | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $			100		99
Fair Values | Mortgage servicing rights | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $			$ 96		$ 93
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Minimum
Inputs:
Input	5.9		5.9		5.8
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Maximum
Inputs:
Input	15.8		15.8		16.1
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Weighted average
Inputs:
Input	8.5		8.5		8.7
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	9.8		9.8		8.4
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	33.0		33.0		32.6
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	15.6		15.6		9.9
Fair Values | Mortgage servicing rights | Life | Level 3 | Minimum
Inputs:
Input	1.4		1.4		1.5
Fair Values | Mortgage servicing rights | Life | Level 3 | Maximum
Inputs:
Input	7.2		7.2		7.9
Fair Values | Mortgage servicing rights | Life | Level 3 | Weighted average
Inputs:
Input	5.2		5.2		7.2
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
Inputs
Amount recognized | $	$ 246,757,000	149,000,000
Unpaid principal balance of underlying loans | $	$ 15,709,249,000	$ 10,790,398,000
Weighted-average servicing fee rate (as a percent)	0.43%	0.37%
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $	$ 78	$ 78
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $	100	98
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $	$ 97	$ 92
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Minimum
Inputs:
Input	5.5	7.3	5.5	7.3
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input	16.2	13.6	16.2	13.6
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Weighted average
Inputs:
Input	8.3	10.1	8.3	10.1
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	8.8	4.4	8.8	4.4
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	32.1	55.7	32.1	55.7
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	15.7	11.8	15.7	11.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input	2.7	0.5	2.7	0.5
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input	7.5	11.3	7.5	11.3
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input	5.5	6.9	5.5	6.9
Fair Values | Mortgage servicing rights | Level 3
Inputs
Amount recognized | $			$ 545,839,000,000	$ 448,604,000,000
Unpaid principal balance of underlying loans | $			$ 35,532,425,000	$ 32,095,458,000
Weighted-average servicing fee rate (as a percent)			4200.00%	3600.00%
Fair Values | Mortgage servicing rights | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $			$ 78	$ 78
Fair Values | Mortgage servicing rights | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $			100	98
Fair Values | Mortgage servicing rights | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $			$ 97	$ 90
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Minimum
Inputs:
Input	5.5	7.3	5.5	7.3
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Maximum
Inputs:
Input	16.2	14.1	16.2	14.1
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Weighted average
Inputs:
Input	8.6	10.2	8.6	10.2
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	7.7	3.9	7.7	3.9
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	32.8	61.8	32.8	61.8
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	15.0	10.6	15.0	10.6
Fair Values | Mortgage servicing rights | Life | Level 3 | Minimum
Inputs:
Input	2.6	0.5	2.6	0.5
Fair Values | Mortgage servicing rights | Life | Level 3 | Maximum
Inputs:
Input	7.8	11.6	7.8	11.6
Fair Values | Mortgage servicing rights | Life | Level 3 | Weighted average
Inputs:
Input	5.8	7.5	5.8	7.5