Agency Mortgage-Backed Securities - Weighted Average Life (Detail) - Agency Mortgage Backed Securities - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value of Agency Mortgage-Backed Securities according to their estimated weighted-average life:
Less than one year	$ 26,908	$ 43,248
Greater than one year through five years	24,579,949	42,222,114
Greater than five years through ten years	40,978,447	39,018,833
Greater than ten years	221,336	281,061
Total	65,806,640	81,565,256	[1]
Amortized Cost of Agency Mortgage-Backed Securities according to their estimated weighted-average life:
Less than one year	27,387	42,831
Greater than one year through five years	24,237,266	41,908,586
Greater than five years through ten years	41,081,291	39,098,352
Greater than ten years	230,740	283,756
Total	$ 65,576,684	$ 81,333,525

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