Summary of Characteristics of Interest Rate Swaptions (Detail) (Interest Rate Swaption, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Long
Derivative Instruments:
Current Underlying Notional	$ 1,750,000	$ 5,150,000
Underlying Pay Rate	2.88	3.07
Underlying Receive Rate	3M LIBOR	3M LIBOR
Years to Maturity	9 years 2 months 1 day	10 years 1 month 6 days
Months to Expiration	3 years 7 months 2 days	4 years 3 months 4 days
Short
Derivative Instruments:
Current Underlying Notional		$ 1,000,000
Underlying Pay Rate	-	3M LIBOR
Underlying Receive Rate	-	2.83
Years to Maturity		5 years 11 months 16 days
Months to Expiration		23 years 8 months 16 days