Estimated Fair Value for All Financial Assets and Liabilities (Detail) - Entity [Domain] - USD ($) $ in Thousands		Jun. 30, 2015		Dec. 31, 2014		Jan. 31, 2014
Financial assets:
Agency mortgage-backed securities		$ 67,605,287		$ 81,565,256
Agency debentures		429,845		1,368,350	[1]
Agency CRT securities		214,130
Commercial real estate debt investments, at fair value	[2]	2,812,824
Investment in affiliate		123,343		143,045	[1]
Interest rate swaps		68,333		80,724
Other derivatives		38,074		5,499
Financial liabilities:
Other secured financing		203,200
Securitized debt of consolidated VIE		2,610,974	[3]	260,700	[1],[3]	$ 260,700
Interest rate swaps		1,369,268		1,616,313
Other derivatives		40,539		8,027
Level 1
Financial assets:
Investment in affiliate		123,343		143,045
Other derivatives		7,734		117
Financial liabilities:
Other derivatives		16,162		3,769
Level 2
Financial assets:
Agency mortgage-backed securities		67,605,287		81,565,256
Agency debentures		429,845		1,368,350
Agency CRT securities		214,130
Commercial real estate debt investments, at fair value		2,812,824
Other derivatives		30,340		5,382
Financial liabilities:
Other derivatives		24,377		4,258
Estimate of Fair Value, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents		1,785,158		1,741,244
Reverse repurchase agreements				100,000
Investment in affiliate		123,343		143,045
Financial liabilities:
Convertible Senior Notes				863,470
Estimate of Fair Value, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities		67,605,287		81,565,256
Agency debentures		429,845		1,368,350
Agency CRT securities		214,130
Commercial real estate debt investments, at fair value		2,812,824
Corporate debt		309,709		166,056
Interest rate swaps		30,259		75,225
Financial liabilities:
Other secured financing		203,376
Securitized debt of consolidated VIE		2,610,974		262,061
Mortgages payable		147,149		146,611
Interest rate swaps		1,328,729		1,608,286
Estimate of Fair Value, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity, held for investment		1,339,843		1,528,444
Financial liabilities:
Participation sold		13,483		13,655
Estimate of Fair Value, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives		38,074		5,499
Financial liabilities:
Repurchase agreements		57,639,127		71,587,222
Other derivatives		40,539		8,027
Carrying (Reported) Amount, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents		1,785,158		1,741,244
Reverse repurchase agreements				100,000
Investment in affiliate		123,343		143,045
Financial liabilities:
Convertible Senior Notes				845,295
Carrying (Reported) Amount, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities		67,605,287		81,565,256
Agency debentures		429,845		1,368,350
Agency CRT securities		214,130
Commercial real estate debt investments, at fair value		2,812,824
Corporate debt		311,640		166,464
Interest rate swaps		30,259		75,225
Financial liabilities:
Other secured financing		203,200
Securitized debt of consolidated VIE		2,610,974		260,700
Mortgages payable		146,359		146,553
Interest rate swaps		1,328,729		1,608,286
Carrying (Reported) Amount, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity, held for investment		1,332,955		1,518,165
Financial liabilities:
Participation sold		13,490		13,693
Carrying (Reported) Amount, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives		38,074		5,499
Financial liabilities:
Repurchase agreements		57,459,552		71,361,926
Other derivatives		$ 40,539		$ 8,027

[1] Derived from the audited consolidated financial statements at December 31, 2014.

[2] Includes senior securitized commercial mortgage loans of consolidated VIEs carried at fair value of $2.6 billion and $0 at June 30, 2015 and December 31, 2014, respectively.

[3] Includes securitized debt of consolidated VIEs carried at fair value of $2.4 billion and $0 at June 30, 2015 and December 31, 2014, respectively.