Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Rights - Initial Recognition (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($) item	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($) item	Dec. 31, 2017 USD ($)
Mortgage servicing rights
MSR and pool characteristics
Amount recognized | $	$ 149,000,000	$ 5,773,000	$ 448,604,000	$ 19,702,000
Fair Values | Mortgage servicing rights | Level 3
MSR and pool characteristics
Weighted-average servicing fee rate (as a percent)			0.32%		0.32%
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 | $			98		97
Fair Values | Mortgage servicing rights | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $			93		$ 89
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
MSR and pool characteristics
Amount recognized | $	149,000,000	5,773,000	448,604,000,000	19,702,000,000
Unpaid principal balance of underlying mortgage loans | $	$ 10,790,398,000	$ 573,463,000	$ 32,095,458,000,000	$ 1,873,404,000,000
Weighted-average servicing fee rate (as a percent)	0.37%	0.31%	0.36%	0.31%
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $	$ 78	$ 78	$ 78	$ 78
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $	98	98	98	101
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $	$ 92	$ 89	$ 90	$ 89
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Minimum
Inputs:
Input	7.3	7.6	7.3	7.6
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input	13.6	11.2	14.1	11.2
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Weighted average
Inputs:
Input	10.1	10.7	10.2	10.5
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	4.4	3.9	3.9	3.9
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	55.7	46.8	61.8	71.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	11.8	13.3	10.6	12.5
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input	0.5	1.4	0.5	0.8
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input	11.3	11.4	11.6	11.5
Fair Values | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input	6.9	6.3	7.5	6.6
Amortized cost | Mortgage servicing rights | Level 3
MSR and pool characteristics
Weighted-average servicing fee rate (as a percent)					0.31%
Amortized cost | Mortgage servicing rights | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $					$ 79
Amortized cost | Mortgage servicing rights | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $					97
Amortized cost | Mortgage servicing rights | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $					$ 89
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
MSR and pool characteristics
Amount recognized | $		$ 153,061,000		$ 412,206,000,000
Unpaid principal balance of underlying mortgage loans | $		$ 12,184,003,000		$ 33,890,209,000,000
Weighted-average servicing fee rate (as a percent)		0.32%		0.30%
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Minimum
Inputs:
Annual per-loan cost of servicing | $		$ 79		$ 79
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Maximum
Inputs:
Annual per-loan cost of servicing | $		98		101
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $		$ 89		$ 89
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Minimum
Inputs:
Input		7.6		7.6
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input		14.6		15.2
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Weighted average
Inputs:
Input		10.8		10.7
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input		4.4		3.4
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input		47.6		47.6
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input		9.5		9.1
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input		1.5		1.5
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input		11.4		12.2
Amortized cost | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input		7.9		8.1