Secured Debt Agreements, Net - Credit Facilities (Detail) - Credit Facilities [Member]	Dec. 31, 2018 USD ($)	Dec. 31, 2018 EUR (€)	Dec. 31, 2018 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 11,030,045,000			$ 7,155,975,000
Potential	7,242,096,000			4,664,097,000
Repurchase Borrowings Outstanding	6,903,683,000			4,068,249,000
Available	338,413,000			595,848,000
Collateral Assets	9,252,152,000			6,064,983,000
Bank of America [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,000,000,000			750,000,000
Potential	873,446,000			573,542,000
Repurchase Borrowings Outstanding	873,446,000			573,542,000
Collateral Assets	1,090,117,000			765,049,000
Wells Fargo [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	3,000,000,000			2,000,000,000
Potential	1,398,103,000			1,289,135,000
Repurchase Borrowings Outstanding	1,311,749,000			1,170,801,000
Available	86,354,000			118,334,000
Collateral Assets	1,853,770,000			1,680,325,000
MetLife [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	704,325,000			1,000,000,000
Potential	675,329,000			807,164,000
Repurchase Borrowings Outstanding	675,329,000			807,164,000
Collateral Assets	852,733,000			1,039,231,000
Barclays [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,500,000,000
Potential	890,620,000
Repurchase Borrowings Outstanding	890,620,000
Collateral Assets	1,113,275,000
Citibank [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,000,000,000			800,125,000
Potential	817,993,000			354,354,000
Repurchase Borrowings Outstanding	630,454,000			240,881,000
Available	187,539,000			113,473,000
Collateral Assets	1,030,116,000			455,433,000
JP Morgan [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			500,000,000
Potential	492,349,000			443,496,000
Repurchase Borrowings Outstanding	492,349,000			319,755,000
Available				123,741,000
Collateral Assets	626,897,000			579,218,000
Morgan Stanley [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	637,700,000		£ 500,000,000	675,650,000
Potential	341,241,000			456,344,000
Repurchase Borrowings Outstanding	276,721,000			216,044,000
Available	64,520,000			240,300,000
Collateral Assets	457,496,000			591,168,000
Deutsche Bank [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,100,000,000			500,000,000
Potential	934,631,000			295,743,000
Repurchase Borrowings Outstanding	934,631,000			295,743,000
Collateral Assets	1,193,713,000			399,203,000
Goldman Sachs [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	450,000,000
Potential	230,140,000
Repurchase Borrowings Outstanding	230,140,000
Collateral Assets	295,368,000
Societe Generale [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	688,020,000	€ 600,000,000		480,200,000	€ 400,000,000
Potential	321,182,000			300,871,000
Repurchase Borrowings Outstanding	321,182,000			300,871,000
Collateral Assets	404,048,000			373,181,000
Goldman Sachs - Multi. JV [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	250,000,000			250,000,000
Potential	170,060,000			57,888,000
Repurchase Borrowings Outstanding	170,060,000			57,888,000
Collateral Assets	212,983,000			75,225,000
Bank of America - Multi. JV [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	200,000,000			200,000,000
Potential	97,002,000			85,560,000
Repurchase Borrowings Outstanding	97,002,000			85,560,000
Collateral Assets	$ 121,636,000			$ 106,950,000