Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Rights - Initial Recognition (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Mortgage servicing rights
Inputs
Amount recognized | $	$ 884,876,000	$ 591,757,000	$ 24,471,000
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 | $	$ 97	$ 93
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Minimum
Inputs:
Input	6.8	5.8
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Maximum
Inputs:
Input	15.8	16.1
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Weighted average
Inputs:
Input	8.5	8.7
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	9.3	8.4
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	40.9	32.6
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	12.7	9.9
Fair Values | Mortgage servicing rights | Life | Level 3 | Minimum
Inputs:
Input	1.4	1.5
Fair Values | Mortgage servicing rights | Life | Level 3 | Maximum
Inputs:
Input	7.4	7.9
Fair Values | Mortgage servicing rights | Life | Level 3 | Weighted average
Inputs:
Input	6.1	7.2
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
Inputs
Amount recognized | $	$ 884,876,000	$ 591,757,000	24,471,000
Unpaid principal balance of underlying loans | $	$ 56,038,354,000	$ 42,008,585,000	$ 2,316,539,000
Weighted-average servicing fee rate (as a percent)	0.41%	0.36%	0.31%
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $	$ 78	$ 78	$ 78
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $	100	99	101
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $	$ 97	$ 91	$ 89
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Minimum
Inputs:
Input	5.5	5.8	7.6
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input	16.2	16.4	11.2
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Weighted average
Inputs:
Input	8.5	9.9	10.5
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	7.7	3.9	3.9
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	32.8	61.8	71.8
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	13.5	10.8	12.5
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input	2.6	0.5	0.8
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input	8.2	11.6	11.5
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input	6.2	7.3	6.6
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
Inputs
Amount recognized | $			$ 556,630,000
Unpaid principal balance of underlying loans | $			$ 44,664,551,000
Weighted-average servicing fee rate (as a percent)			0.31%
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $			$ 79
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $			101
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $			$ 89
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Minimum
Inputs:
Input			7.6
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input			15.2
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Weighted average
Inputs:
Input			10.7
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input			3.4
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input			47.6
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input			9.1
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input			1.5
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input			12.2
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input			8.1