Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance accumulated other comprehensive income	$ 269,048		$ 262,854
Other comprehensive income (loss) before reclassification	(604,697)		(619,263)
Amount reclassified to mortgage-backed securities available for sale	(6,968)		(13,637)
Amounts reclassified to net gain on the sale of mortgage-backed securities	(8,802)		(11,302)
Amounts reclassified to interest expense	30,586		60,515
Net current period other comprehensive income (loss)	(589,881)	13,253	(583,687)	40,338
Ending balance accumulated other comprehensive income (loss)	(320,833)		(320,833)
Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance accumulated other comprehensive income	481,323		499,343
Other comprehensive income (loss) before reclassification	(615,199)		(630,719)
Amount reclassified to mortgage-backed securities available for sale	(8,802)		(11,302)
Net current period other comprehensive income (loss)	(624,001)		(642,021)
Ending balance accumulated other comprehensive income (loss)	(142,678)		(142,678)
Unsettled Agency Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance accumulated other comprehensive income	492		1,217
Other comprehensive income (loss) before reclassification	(2,776)		(2,284)
Amount reclassified to mortgage-backed securities available for sale	(492)		(1,709)
Net current period other comprehensive income (loss)	(3,268)		(3,993)
Ending balance accumulated other comprehensive income (loss)	(2,776)		(2,776)
Other Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance accumulated other comprehensive income	(214)		(107)
Other comprehensive income (loss) before reclassification	(291)		(398)
Net current period other comprehensive income (loss)	(291)		(398)
Ending balance accumulated other comprehensive income (loss)	(505)		(505)
Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(212,553)		(237,599)
Other comprehensive income (loss) before reclassification	13,569		14,138
Amount reclassified to mortgage-backed securities available for sale	(6,476)		(11,928)
Amounts reclassified to interest expense	30,586		60,515
Net current period other comprehensive income (loss)	37,679		62,725
Ending balance	$ (174,874)		$ (174,874)