Estimated Fair Value for All Financial Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Jan. 31, 2014
Financial assets:
Agency mortgage-backed securities	$ 69,388,001		$ 81,565,256
Agency debentures	995,408		1,368,350
Agency CRT securities	108,337
Commercial real estate debt investments	1,515,903	[1]
Investment in affiliates	141,246		143,045
Interest rate swaps	139,411		80,724
Other derivatives	113,503		5,499
Financial liabilities:
Securitized debt of consolidated VIE	1,491,829	[2]	260,700	[2],[3]	260,700
Interest rate swaps	2,086,948		1,616,313
Other derivatives	61,778		8,027
Level 1
Financial assets:
Investment in affiliates	141,246		143,045
Other derivatives			117
Financial liabilities:
Other derivatives	61,778		3,769
Level 2
Financial assets:
Agency mortgage-backed securities	69,388,001		81,565,256
Agency debentures	995,408		1,368,350
Agency CRT securities	108,337
Commercial real estate debt investments	1,515,903
Other derivatives	113,503		5,382
Financial liabilities:
Other derivatives			4,258
Estimate of Fair Value, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	1,920,326		1,741,244
Reverse repurchase agreements	100,000		100,000
Investment in affiliates	141,246		143,045
Financial liabilities:
Convertible Senior Notes	752,325		863,470
Estimate of Fair Value, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities	69,388,001		81,565,256
Agency debentures	995,408		1,368,350
Agency CRT securities	108,337
Commercial real estate debt investments	1,515,903
Corporate debt	228,012		166,056
Interest rate swaps	25,908		75,225
Financial liabilities:
Securitized debt of consolidated VIE	1,492,102		262,061
Mortgages payable	150,765		146,611
Interest rate swaps	2,025,170		1,608,286
Estimate of Fair Value, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity	1,513,878		1,528,444
Financial liabilities:
Participation sold	13,620		13,655
Estimate of Fair Value, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives	113,503		5,499
Financial liabilities:
Repurchase agreements	60,691,054		71,587,222
Other derivatives	61,778		8,027
Carrying (Reported) Amount, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	1,920,326		1,741,244
Reverse repurchase agreements	100,000		100,000
Investment in affiliates	141,246		143,045
Financial liabilities:
Convertible Senior Notes	749,512		845,295
Carrying (Reported) Amount, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities	69,388,001		81,565,256
Agency debentures	995,408		1,368,350
Agency CRT securities	108,337
Commercial real estate debt investments	1,515,903
Corporate debt	227,830		166,464
Interest rate swaps	25,908		75,225
Financial liabilities:
Securitized debt of consolidated VIE	1,491,829		260,700
Mortgages payable	146,470		146,553
Interest rate swaps	2,025,170		1,608,286
Carrying (Reported) Amount, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity	1,498,406		1,518,165
Financial liabilities:
Participation sold	13,589		13,693
Carrying (Reported) Amount, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives	113,503		5,499
Financial liabilities:
Repurchase agreements	60,477,378		71,361,926
Other derivatives	$ 61,778		$ 8,027

[1] Includes senior securitized commercial mortgage loans of a consolidated VIE carried at fair value of $1.4 billion and $0 at March 31, 2015 and December 31, 2014, respectively.

[2] Includes securitized debt of a consolidated VIE carried at fair value of $1.3 billion and $0 at March 31, 2015 and December 31, 2014, respectively.

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