Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Rights, Effect of Change In Inputs on Fair Value (Details) - Level 3	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Mortgage servicing liabilities
MSR and pool characteristics
Carrying value	$ 9,769,000	$ 14,120,000
Unpaid principal balance of underlying mortgage loans	$ 1,265,461,000	$ 1,620,609,000
Weighted-average servicing fee rate (as a percent)	25.00%	0.25%
Prepayment speed
Annual per-loan cost of servicing	$ 372	$ 404
Mortgage servicing liabilities | Pricing spread
Inputs
Input | item	7.8	7.7
Mortgage servicing liabilities | Annual total prepayment speed
Inputs
Input | item	31.6	32.9
Mortgage servicing liabilities | Life
Inputs
Input | item	3.9	3.5
Fair Values | Mortgage servicing rights
MSR and pool characteristics
Carrying value	$ 2,785,964,000	$ 638,010,000
Unpaid principal balance of underlying mortgage loans	$ 193,659,378,000	$ 51,883,539,000
Weighted-average note interest rate (as a percent)	3.90%	4.00%
Weighted-average servicing fee rate (as a percent)	0.32%	0.32%
Pricing spread
Effect on fair value of 5% adverse change	$ (51,013,000)	$ (10,760,000)
Effect on fair value of 10% adverse change	(100,182,000)	(21,155,000)
Effect on fair value of 20% adverse change	(193,355,000)	(40,916,000)
Prepayment speed
Effect on fair value of 5% adverse change	(36,453,000)	(10,809,000)
Effect on fair value of 10% adverse change	(71,779,000)	(21,239,000)
Effect on fair value of 20% adverse change	(139,263,000)	(41,038,000)
Annual per-loan cost of servicing
Effect on fair value of 5% adverse change	(22,201,000)	(6,247,000)
Effect on fair value of 10% adverse change	(44,402,000)	(12,494,000)
Effect on fair value of 20% adverse change	(88,803,000)	(24,987,000)
Fair Values | Mortgage servicing rights | Minimum
Prepayment speed
Annual per-loan cost of servicing	78	78
Fair Values | Mortgage servicing rights | Maximum
Prepayment speed
Annual per-loan cost of servicing	98	97
Fair Values | Mortgage servicing rights | Weighted average
Prepayment speed
Annual per-loan cost of servicing	$ 93	$ 89
Fair Values | Mortgage servicing rights | Pricing spread | Minimum
Inputs
Input | item	7.3	7.6
Fair Values | Mortgage servicing rights | Pricing spread | Maximum
Inputs
Input | item	13.6	14.1
Fair Values | Mortgage servicing rights | Pricing spread | Weighted average
Inputs
Input | item	10.0	9.8
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Minimum
Inputs
Input | item	6.9	7.9
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Maximum
Inputs
Input | item	41.6	46.2
Fair Values | Mortgage servicing rights | Annual total prepayment speed | Weighted average
Inputs
Input | item	8.2	10.5
Fair Values | Mortgage servicing rights | Life | Minimum
Inputs
Input | item	1.3	1.2
Fair Values | Mortgage servicing rights | Life | Maximum
Inputs
Input | item	8.5	7.8
Fair Values | Mortgage servicing rights | Life | Weighted average
Inputs
Input | item	8.1	6.6
Amortized cost | Mortgage servicing rights
MSR and pool characteristics
Carrying value		$ 1,481,578,000
Unpaid principal balance of underlying mortgage loans		$ 114,365,698,000
Weighted-average note interest rate (as a percent)		3.80%
Weighted-average servicing fee rate (as a percent)		0.31%
Pricing spread
Effect on fair value of 5% adverse change		$ (27,700,000)
Effect on fair value of 10% adverse change		(54,376,000)
Effect on fair value of 20% adverse change		(104,869,000)
Prepayment speed
Effect on fair value of 5% adverse change		(23,544,000)
Effect on fair value of 10% adverse change		(46,284,000)
Effect on fair value of 20% adverse change		(89,514,000)
Annual per-loan cost of servicing
Effect on fair value of 5% adverse change		(11,216,000)
Effect on fair value of 10% adverse change		(22,431,000)
Effect on fair value of 20% adverse change		(44,863,000)
Amortized cost | Mortgage servicing rights | Minimum
Prepayment speed
Annual per-loan cost of servicing		79
Amortized cost | Mortgage servicing rights | Maximum
Prepayment speed
Annual per-loan cost of servicing		97
Amortized cost | Mortgage servicing rights | Weighted average
Prepayment speed
Annual per-loan cost of servicing		$ 89
Amortized cost | Mortgage servicing rights | Pricing spread | Minimum
Inputs
Input | item		7.6
Amortized cost | Mortgage servicing rights | Pricing spread | Maximum
Inputs
Input | item		14.1
Amortized cost | Mortgage servicing rights | Pricing spread | Weighted average
Inputs
Input | item		10.3
Amortized cost | Mortgage servicing rights | Annual total prepayment speed | Minimum
Inputs
Input | item		7.4
Amortized cost | Mortgage servicing rights | Annual total prepayment speed | Maximum
Inputs
Input | item		44.1
Amortized cost | Mortgage servicing rights | Annual total prepayment speed | Weighted average
Inputs
Input | item		9.7
Amortized cost | Mortgage servicing rights | Life | Minimum
Inputs
Input | item		2.0
Amortized cost | Mortgage servicing rights | Life | Maximum
Inputs
Input | item		8.3
Amortized cost | Mortgage servicing rights | Life | Weighted average
Inputs
Input | item		7.5