Fair Value - Summary of Changes in Fair Value Included in Current Period Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		$ (9,815)		$ (32,652)
Credit Risk Transfer Strips
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	$ 5,675		$ 5,675
Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		0		0
Net Loan Servicing Fees [Member] | Credit Risk Transfer Strips
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0		0
Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		0		0
Net Gain on Loans Acquired for Sale [Member] | Credit Risk Transfer Strips
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0		0
Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		(8,861)		(31,258)
Net Gain (Loss) on Investments [Member] | Credit Risk Transfer Strips
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	5,675		5,675
Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		(954)		(1,394)
Net Interest Income [Member] | Credit Risk Transfer Strips
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0		0
Interest-only security payable [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	6,208	144	9,655	(582)
Interest-only security payable [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Interest-only security payable [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Interest-only security payable [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	6,208	144	9,655	(582)
Interest-only security payable [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Asset-Backed Financing of the VIE at Fair Value [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(3,525)	2,747	(7,203)	9,268
Asset-Backed Financing of the VIE at Fair Value [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Asset-Backed Financing of the VIE at Fair Value [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Asset-Backed Financing of the VIE at Fair Value [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(2,341)	2,960	(5,198)	9,142
Asset-Backed Financing of the VIE at Fair Value [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(1,184)	(213)	(2,005)	126
Liabilities, Total [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	2,683	2,891	2,452	8,686
Liabilities, Total [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Liabilities, Total [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Liabilities, Total [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	3,867	3,104	4,457	8,560
Liabilities, Total [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(1,184)	(213)	(2,005)	126
Short-term investments at fair value [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Short-term investments at fair value [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Short-term investments at fair value [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Short-term investments at fair value [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Short-term investments at fair value [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Mortgage-backed securities at fair value [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	21,234		53,600
Mortgage-backed securities at fair value [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0		0
Mortgage-backed securities at fair value [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0		0
Mortgage-backed securities at fair value [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	27,448		64,370
Mortgage-backed securities at fair value [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(6,214)		(10,770)
Loans acquired for sale at fair value [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	54,790	(4,470)	92,593	(28,148)
Loans acquired for sale at fair value [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Loans acquired for sale at fair value [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	54,790	(4,470)	92,593	(28,148)
Loans acquired for sale at fair value [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Loans acquired for sale at fair value [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Loans at fair value [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(725)	(5,208)	4,257	(18,962)
Loans at fair value [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Loans at fair value [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Loans at fair value [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(2,092)	(7,485)	1,978	(23,013)
Loans at fair value [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	1,367	2,277	2,279	4,051
Firm commitment to purchase CRT securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	24,526		66,316	4,426
Firm commitment to purchase CRT securities [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0		0	0
Firm commitment to purchase CRT securities [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	20,396		39,996	4,426
Firm commitment to purchase CRT securities [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	4,130		26,320	0
Firm commitment to purchase CRT securities [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0		0	0
Excess servicing spread [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(837)	4,906	(1,822)	15,761
Excess servicing spread [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Excess servicing spread [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Excess servicing spread [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(3,604)	996	(7,655)	7,917
Excess servicing spread [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	2,767	3,910	5,833	7,844
MSRs at fair value [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(183,467)	(11,914)	(320,796)	14,059
MSRs at fair value [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(183,467)	(11,914)	(320,796)	14,059
MSRs at fair value [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
MSRs at fair value [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
MSRs at fair value [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	0	0	0	0
Assets, Total [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(78,804)	(22,075)	(100,177)	(45,516)
Assets, Total [Member] | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	(183,467)	(11,914)	(320,796)	14,059
Assets, Total [Member] | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	75,186	(44)	132,589	(23,722)
Assets, Total [Member] | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	31,557	(15,350)	90,688	(46,354)
Assets, Total [Member] | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income	$ (2,080)	5,233	$ (2,658)	$ 10,501
Firm Commitment To Purchase Credit Risk Transfer Security
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		4,426
Firm Commitment To Purchase Credit Risk Transfer Security | Net Loan Servicing Fees [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		0
Firm Commitment To Purchase Credit Risk Transfer Security | Net Gain on Loans Acquired for Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		4,426
Firm Commitment To Purchase Credit Risk Transfer Security | Net Gain (Loss) on Investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		0
Firm Commitment To Purchase Credit Risk Transfer Security | Net Interest Income [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in fair value included in current period income		$ 0