Secured Debt Agreements, Net - Credit Facilities (Detail)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 EUR (€)	Jun. 30, 2018 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 GBP (£)
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 660,400,000		£ 500,000,000	$ 675,700,000		£ 500,000,000
Potential	5,384,738,000
Repurchase Borrowings Outstanding	4,649,246,000
Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	8,327,710,000			7,155,975,000
Potential	5,384,738,000			4,664,097,000
Repurchase Borrowings Outstanding	4,649,246,000			4,068,249,000
Available	735,493,000			595,848,000
Collateral Assets	7,009,195,000			6,064,983,000
Bank of America [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,000,000,000			750,000,000
Potential	863,592,000			573,542,000
Repurchase Borrowings Outstanding	863,592,000			573,542,000
Collateral Assets	1,083,385,000			765,049,000
Wells Fargo [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	2,000,000,000			2,000,000,000
Potential	1,348,240,000			1,289,135,000
Repurchase Borrowings Outstanding	852,431,000			1,170,801,000
Available	495,809,000			118,334,000
Collateral Assets	1,822,275,000			1,680,325,000
MetLife [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,000,000,000			1,000,000,000
Potential	793,130,000			807,164,000
Repurchase Borrowings Outstanding	793,130,000			807,164,000
Collateral Assets	1,041,540,000			1,039,231,000
Barclays [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,000,000,000
Potential	496,480,000
Repurchase Borrowings Outstanding	496,480,000
Collateral Assets	620,600,000
Citibank [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	750,000,000			800,125,000
Potential	572,341,000			354,354,000
Repurchase Borrowings Outstanding	459,827,000			240,881,000
Available	112,514,000			113,473,000
Collateral Assets	719,177,000			455,433,000
JP Morgan [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			500,000,000
Potential	359,114,000			443,496,000
Repurchase Borrowings Outstanding	296,058,000			319,755,000
Available	63,056,000			123,741,000
Collateral Assets	474,407,000			579,218,000
Morgan Stanley [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	660,350,000			675,650,000
Potential	344,960,000			456,344,000
Repurchase Borrowings Outstanding	280,847,000			216,044,000
Available	64,114,000			240,300,000
Collateral Assets	460,991,000			591,168,000
Deutsche Bank [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			500,000,000
Potential	277,247,000			295,743,000
Repurchase Borrowings Outstanding	277,247,000			295,743,000
Collateral Assets	373,042,000			399,203,000
Societe Generale [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	467,360,000	€ 400,000,000		480,200,000	€ 400,000,000
Potential	235,229,000			300,871,000
Repurchase Borrowings Outstanding	235,229,000			300,871,000
Collateral Assets	294,654,000			373,181,000
Goldman Sachs - Multi. JV [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	250,000,000			250,000,000
Potential	55,085,000			57,888,000
Repurchase Borrowings Outstanding	55,085,000			57,888,000
Collateral Assets	69,974,000			75,225,000
Bank of America - Multi. JV [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	200,000,000			200,000,000
Potential	39,320,000			85,560,000
Repurchase Borrowings Outstanding	39,320,000			85,560,000
Collateral Assets	$ 49,150,000			$ 106,950,000