Fair Value Measurements - Unobservable Input Reconciliation (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Fair Value Assets (Liabilities) Measured On Recurring Basis, Unobservable Input Reconciliation Calculation [Roll Forward]
Total, Beginning balance, Level 3	$ 5,142,619	$ 4,723,761
Total, Purchases	733,532	358,029
Total, Principal payments	(121,490)	(495,256)
Total, Sales and Settlements	(18,774)	463,885
Total, Accretion (amortization) of purchase discounts	23,027	77,147
Total, Other than temporary credit impairment losses	(16,726)	(57,986)
Total, Realized gains (losses) on sales and settlements	5,150	13,639
Total, Net unrealized gains (losses) included in income	72,992	60,044
Total, Total unrealized gains (losses) for the period	3,929	(644)
Total, Ending balance, Level 3	5,830,371	5,142,619
Securitized Debt | Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure
Fair Value, Liabilities Measured on Recurring Basis [Roll Forward]
Securitized Debt, Beginning balance, Level 3	(6,941,097)	(3,720,496)
Securitized Debt, Transfers in to Level 3 assets	0	0
Securitized Debt, Transfers out of Level 3 assets	0	0
Securitized Debt, Purchases	(3,457,535)	(4,797,255)
Securitized Debt, Principal payments	331,290	1,059,854
Securitized Debt, Sales and Settlements	0	608,816
Securitized Debt, Accretion (amortization) of purchase discounts	(4,525)	(2,128)
Securitized Debt, Realized gains (losses) on sales and settlements	0	(122)
Securitized Debt, Net unrealized gains (losses) included in income	(39,426)	(89,766)
Securitized Debt, Gains (losses) included in other comprehensive income	0	0
Securitized Debt, Ending balance, Level 3	(10,111,293)	(6,941,097)
Non-agency MBS
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Beginning balance Level 3 assets	3,330,063	3,675,841
Transfers in to Level 3 assets	6,112	0
Transfers out of Level 3 assets	0	0
Purchases	25,745	257,914
Principal payments	(127,929)	(532,696)
Sales and Settlements	(20,063)	(149,938)
Accretion (amortization) of purchase discounts	30,661	120,638
Other than temporary credit impairment losses	(16,726)	(57,986)
Realized gains (losses) on sales and settlements	5,150	13,761
Net unrealized gains (losses) included in income	(8,551)	3,173
Total unrealized gains (losses) for the period	3,929	(644)
Ending balance Level 3 assets	3,228,391	3,330,063
Securitized Loans | Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Beginning balance Level 3 assets	8,753,653	4,768,416
Transfers in to Level 3 assets	0	0
Transfers out of Level 3 assets	0	0
Purchases	4,165,322	4,897,370
Principal payments	(324,851)	(1,022,414)
Sales and Settlements	1,289	5,007
Accretion (amortization) of purchase discounts	(3,109)	(41,363)
Net unrealized gains (losses) included in income		146,637
Change in fair value	120,969	146,637
Total unrealized gains (losses) for the period	0	0
Ending balance Level 3 assets	$ 12,713,273	$ 8,753,653