Repurchase Agreements - Narrative (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Repurchase Agreements:
Repurchase agreements - outstanding	$ 60,477,378,000	$ 71,361,926,000	[1]
Repurchase agreements - weighted average borrowing rates	1.74%	1.62%
Repurchase agreements - weighted average remaining maturities (in days)	149 days	141 days
Secured financings and interest rate swaps - collateral held, estimated fair value	64,600,000,000	75,400,000,000
Secured financings and interest rate swaps - collateral held, accrued interest	$ 189,900,000	$ 226,600,000

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