Repurchase Agreements - Narrative (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Repurchase Agreements:
Repurchase agreements - outstanding	$ 56,449,364	$ 71,361,926	[1]
Repurchase agreements - weighted average borrowing rates	1.75%	1.62%
Repurchase agreements - weighted average remaining maturities (in days)	147 days	141 days
Secured financings and interest rate swaps - collateral held, estimated fair value	$ 60,800,000	$ 75,400,000
Secured financings and interest rate swaps - collateral held, accrued interest	$ 187,200	$ 226,600

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