Secured Debt Agreements, Net - Credit Facilities (Detail)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 GBP (£)	Mar. 31, 2018 EUR (€)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 EUR (€)
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 700,900,000	£ 500,000,000		$ 675,700,000	£ 500,000,000
Potential	4,665,513,000
Repurchase Borrowings Outstanding	3,851,609,000
Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	8,392,900,000			7,155,975,000
Collateral Assets	6,068,268,000			6,064,983,000
Potential	4,665,513,000			4,664,097,000
Repurchase Borrowings Outstanding	3,851,609,000			4,068,249,000
Available	813,904,000			595,848,000
JP Morgan [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			500,000,000
Collateral Assets	558,426,000			579,218,000
Potential	421,250,000			443,496,000
Repurchase Borrowings Outstanding	210,212,000			319,755,000
Available	211,038,000			123,741,000
Societe Generale [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	492,000,000		€ 400,000,000	480,200,000		€ 400,000,000
Collateral Assets	375,742,000			373,181,000
Potential	300,871,000			300,871,000
Repurchase Borrowings Outstanding	300,871,000			300,871,000
Citibank [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	750,000,000			800,125,000
Collateral Assets	548,629,000			455,433,000
Potential	430,290,000			354,354,000
Repurchase Borrowings Outstanding	430,290,000			240,881,000
Available				113,473,000
Wells Fargo [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	2,000,000,000			2,000,000,000
Collateral Assets	1,603,878,000			1,680,325,000
Potential	1,205,597,000			1,289,135,000
Repurchase Borrowings Outstanding	857,498,000			1,170,801,000
Available	348,099,000			118,334,000
MetLife [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,000,000,000			1,000,000,000
Collateral Assets	1,051,140,000			1,039,231,000
Potential	822,534,000			807,164,000
Repurchase Borrowings Outstanding	822,534,000			807,164,000
Bank of America [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,000,000,000			750,000,000
Collateral Assets	820,016,000			765,049,000
Potential	637,992,000			573,542,000
Repurchase Borrowings Outstanding	637,992,000			573,542,000
Deutsche Bank [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			500,000,000
Collateral Assets	360,564,000			399,203,000
Potential	265,643,000			295,743,000
Repurchase Borrowings Outstanding	265,643,000			295,743,000
Morgan Stanley [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	700,900,000			675,650,000
Collateral Assets	594,549,000			591,168,000
Potential	457,971,000			456,344,000
Repurchase Borrowings Outstanding	203,204,000			216,044,000
Available	254,767,000			240,300,000
Bank of America - Multi. JV [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	200,000,000			200,000,000
Collateral Assets	112,550,000			106,950,000
Potential	90,040,000			85,560,000
Repurchase Borrowings Outstanding	90,040,000			85,560,000
Goldman Sachs - Multi. JV [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	250,000,000			250,000,000
Collateral Assets	42,774,000			75,225,000
Potential	33,325,000			57,888,000
Repurchase Borrowings Outstanding	33,325,000			$ 57,888,000
Barclays [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 1,000,000,000