Summary of Characteristics of TBA Derivatives (Detail) (TBA Derivatives, USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Derivative Instruments:
Notional	$ 13,750,000	$ (375,000)
Implied Cost Basis	14,279,766	(375,430)
Implied Market Value	14,392,695	(379,688)
Carrying Value	112,929	(4,258)
Purchase Contracts
Derivative Instruments:
Notional	13,750,000
Implied Cost Basis	14,279,766
Implied Market Value	14,392,695
Carrying Value	112,929
Sales Contracts
Derivative Instruments:
Notional		(375,000)
Implied Cost Basis		(375,430)
Implied Market Value		(379,688)
Carrying Value		$ (4,258)