Residential Investment Securities - Weighted Average Life (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value of Agency Mortgage-Backed Securities according to their estimated weighted-average life:
Less than one year	$ 72,348	$ 37,862
Greater than one year through five years	46,745,799	20,278,111
Greater than five years through ten years	20,238,032	46,473,701
Greater than ten years	199,354	443,820
Total	67,255,533	67,233,494
Amortized Cost of Agency Mortgage-Backed Securities according to their estimated weighted-average life:
Less than one year	73,220	37,850
Greater than one year through five years	46,268,066	20,066,435
Greater than five years through ten years	20,176,740	47,174,319
Greater than ten years	200,350	436,693
Total	$ 66,718,376	$ 67,715,297