Fair Value Measurements - Unobservable Input Reconciliation (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Transfers in to Level 3	$ 0
Transfers out of Level 3	0	$ 0
Securitized Debt | Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Other than temporary credit impairment losses	0	0
Realized gains (losses) on sales and settlements	(26,375,000)	35,273,000
Net unrealized gains (losses) included in income	(52,209,000)	78,289,000
Fair Value, Liabilities Measured on Recurring Basis [Roll Forward]
Securitized Debt, Beginning balance, Level 3	9,388,657,000	6,941,097,000
Securitized Debt, Transfers in to Level 3 assets	0	0
Securitized Debt, Transfers out of Level 3 assets	0	0
Securitized Debt, Purchases	1,769,539,000	5,173,430,000
Securitized Debt, Principal payments	(1,778,625,000)	(1,787,368,000)
Securitized Debt, Sales and Settlements	(834,414,000)	(1,056,765,000)
Securitized Debt, Net accretion (amortization)	(11,197,000)	4,701,000
Securitized Debt, Gains (losses) included in other comprehensive income	0	0
Securitized Debt, Ending balance, Level 3	8,455,376,000	9,388,657,000
Non-agency MBS
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Beginning balance Level 3	2,851,316,000	3,330,063,000
Transfers in to Level 3	0	6,112,000
Transfers out of Level 3	0	0
Purchases of assets/ issuance of debt	253,952,000	78,400,000
Principal payments	(521,109,000)	(568,339,000)
Sales and settlements	(100,786,000)	(68,162,000)
Net accretion (amortization)	101,088,000	120,119,000
Other than temporary credit impairment losses	(21,791,000)	(59,881,000)
Realized gains (losses) on sales and settlements	201,000	12,031,000
Net unrealized gains (losses) included in income	21,208,000	(9,733,000)
Total unrealized gains (losses) for the period	(97,949,000)	10,706,000
Ending balance Level 3	2,486,130,000	2,851,316,000
Loans held for investment | Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Beginning balance Level 3	13,678,263,000	8,753,653,000
Transfers in to Level 3	0	0
Transfers out of Level 3	0	0
Purchases of assets/ issuance of debt	1,671,330,000	6,539,765,000
Principal payments	(1,859,155,000)	(1,788,409,000)
Sales and settlements	(807,364,000)	2,876,000
Net accretion (amortization)	(83,393,000)	(35,803,000)
Other than temporary credit impairment losses	0	0
Realized gains (losses) on sales and settlements	101,000	0
Net unrealized gains (losses) included in income	(27,201,000)	206,181,000
Total unrealized gains (losses) for the period	0	0
Ending balance Level 3	$ 12,572,581,000	$ 13,678,263,000