Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Rights - Initial Recognition (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Mortgage servicing rights
Inputs
Amount recognized | $	$ 591,757,000	$ 24,471,000	$ 17,319,000
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 | $	99	97
Fair Values | Mortgage servicing rights | Level 3 | Weighted average
Inputs:
Annual per-loan cost of servicing | $	$ 93	$ 89
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Minimum
Inputs:
Input	5.8	7.6
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Maximum
Inputs:
Input	16.1	14.1
Fair Values | Mortgage servicing rights | Pricing spread | Level 3 | Weighted average
Inputs:
Input	8.7	9.8
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	8.4	7.9
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	32.6	46.2
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input	9.9	10.5
Fair Values | Mortgage servicing rights | Life | Level 3 | Minimum
Inputs:
Input	1.5	1.2
Fair Values | Mortgage servicing rights | Life | Level 3 | Maximum
Inputs:
Input	7.9	7.8
Fair Values | Mortgage servicing rights | Life | Level 3 | Weighted average
Inputs:
Input	7.2	6.6
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
Inputs
Amount recognized | $	$ 591,757,000	$ 24,471,000	17,319,000
Unpaid principal balance of underlying mortgage loans | $	$ 42,008,585,000	$ 2,316,539,000	$ 1,452,779,000
Weighted-average servicing fee rate (as a percent)	0.36%	0.31%	0.33%
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	$ 68
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 | $	99	101	105
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 | $	$ 91	$ 89	$ 88
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Minimum
Inputs:
Input	5.8	7.6	7.2
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input	16.4	11.2	10.5
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Weighted average
Inputs:
Input	9.9	10.5	9.2
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input	3.9	3.9	3.3
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input	61.8	71.8	53.8
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	10.8	12.5	11.8
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input	0.5	0.8	0.5
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input	11.6	11.5	11.9
Fair Values | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input	7.3	6.6	6.8
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 | Mortgage servicing rights | Pricing spread | Level 3 | Minimum
Inputs:
Input		7.6
Amortized cost | Mortgage servicing rights | Pricing spread | Level 3 | Maximum
Inputs:
Input		14.1
Amortized cost | Mortgage servicing rights | Pricing spread | Level 3 | Weighted average
Inputs:
Input		10.3
Amortized cost | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Minimum
Inputs:
Input		7.4
Amortized cost | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Maximum
Inputs:
Input		44.1
Amortized cost | Mortgage servicing rights | Annual total prepayment speed | Level 3 | Weighted average
Inputs:
Input		9.7
Amortized cost | Mortgage servicing rights | Life | Level 3 | Minimum
Inputs:
Input		2.0
Amortized cost | Mortgage servicing rights | Life | Level 3 | Maximum
Inputs:
Input		8.3
Amortized cost | Mortgage servicing rights | Life | Level 3 | Weighted average
Inputs:
Input		7.5
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Level 3
Inputs
Amount recognized | $		$ 556,630,000	$ 560,212,000
Unpaid principal balance of underlying mortgage loans | $		$ 44,664,551,000	$ 44,827,516,000
Weighted-average servicing fee rate (as a percent)		0.31%	0.30%
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	$ 68
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	106
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	$ 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	7.2
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Pricing spread | Level 3 | Maximum
Inputs:
Input		15.2	14.4
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	9.5
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	2.8
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	50.9
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	9.0
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Minimum
Inputs:
Input		1.5	1.3
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Maximum
Inputs:
Input		12.2	12.9
Amortized cost | Mortgage servicing rights | MSRs at the time of initial recognition, excluding MSR purchases | Life | Level 3 | Weighted average
Inputs:
Input		8.1	8.1