DISCLOSURES REGARDING FAIR VALUES OF FINANCIAL INSTRUMENTS - Fair Value and Related Disclosures for Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Agency Securities Classified as Available-for-sale [Member]
Available-for-sale securities disclosure Items [Abstract]
Available-for-sale securities, Amortized Cost Basis	$ 13,504,096	$ 13,519,657
Available-for-sale securities, Gross Unrealized Gains	244,090	328,412
Available-for-sale securities, Gross Unrealized Losses	21,678	1,963
Available-for-sale securities, Fair Value	13,726,508	13,846,106
Residential Mortgage Securities Classified as held-to-Maturity [Member]
Held-to-maturity securities disclosure [Abstract]
Held-to-maturities, Amortized Cost Basis	4,561	5,989
Held-to-maturities, Gains	224	309
Held-to-maturities, Losses	0	0
Held-to-maturities, Fair Value	$ 4,785	$ 6,298