Repurchase Agreements - Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 1,658,906	$ 2,672,989	[1]
Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	35,430
1-30 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	1,130,037	1,368,945
30 to 59 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	144,595	836,007
90 to 189 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	35,475	171,836
Greater than or equal to 120 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 313,369	$ 296,201

[1] Derived from the audited consolidated financial statements.