Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
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	25,908		75,225	[2]
Other derivatives	113,503		5,499
Total Assets	72,288,306		83,157,375
Liabilities:
Securitized debt of consolidated VIEs	1,268,809		0	[2]
Interest rate swaps	2,025,170		1,608,286	[2]
Other derivatives	61,778		8,027
Total Liabilities	3,355,757		1,616,313
Level 1
Assets:
Investment in affiliates	141,246		143,045
Other derivatives			117
Total Assets	141,246		143,162
Liabilities:
Other derivatives	61,778		3,769
Total Liabilities	61,778		3,769
Level 2
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
Interest rate swaps	25,908		75,225
Other derivatives	113,503		5,382
Total Assets	72,147,060		83,014,213
Liabilities:
Securitized debt of consolidated VIEs	1,268,809
Interest rate swaps	2,025,170		1,608,286
Other derivatives			4,258
Total Liabilities	$ 3,293,979		$ 1,612,544

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