Summary of Fair Value Information about Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Interest rate swaps, at fair value	$ 25,908	$ 75,225	[1]
Other derivatives, at fair value	113,503	5,499	[1]
Derivative assets	139,411	80,724
Other derivatives, at fair value	61,778	8,027	[1]
Interest rate swaps, at fair value	2,025,170	1,608,286	[1]
Derivative liabilities	2,086,948	1,616,313
Future
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value		117
Other derivatives, at fair value	61,778	3,769
Interest Rate Swaption
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	574	5,382
TBA Derivatives
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	112,929
Other derivatives, at fair value		$ 4,258

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