Securities (Details 2) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Fair Value, Available for Sale	$ 2,863,071	$ 7,225,653
Less Than 12 Months, Unrealized Losses, Available for Sale	(8,280)	(97,153)
12 Months or Longer, Fair Value, Available for Sale	0	0
12 Months or Longer, Unrealized Losses, Available for Sale	0	0
Total Available for sale securities, Fair Value	2,863,071	7,225,653
Total Available for sale securities, Unrealized Losses	(8,280)	(97,153)
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value, Held to Maturity	2,908,484	3,532,154
Less Than 12 Months, Unrealized Losses, Held to Maturity	(41,605)	(75,240)
12 Months or Longer, Fair Value, Held to Maturity	0	3,163
12 Months or Longer, Unrealized Losses, Held to Maturity	0	(8)
Total Held to maturity securities, Fair Value	2,908,484	3,535,317
Total Held to maturity securities, Unrealized Losses	(41,605)	(75,248)
Fair Value and Unrealized Losses of Securities
Total Mortgage-backed Securities, Less Than 12 Months, Fair Value	5,771,555	10,757,807
Total Mortgage-backed Securities, Less Than 12 Months, Unrealized Losses	(49,885)	(172,393)
Total Mortgage-backed Securities, 12 Months or Longer, Fair Value	0	3,163
Total Mortgage-backed Securities, 12 Months or Longer, Unrealized Losses	0	(8)
Total Mortgage-backed Securities, Fair Value	5,771,555	10,760,970
Total Mortgage-backed Securities, Unrealized Losses	(49,885)	(172,401)
GNMA pass-through certificates [Member]
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Fair Value, Available for Sale	403,749	424,575
Less Than 12 Months, Unrealized Losses, Available for Sale	(1,114)	(7,487)
12 Months or Longer, Fair Value, Available for Sale	0	0
12 Months or Longer, Unrealized Losses, Available for Sale	0	0
Total Available for sale securities, Fair Value	403,749	424,575
Total Available for sale securities, Unrealized Losses	(1,114)	(7,487)
FNMA pass-through certificates [Member]
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Fair Value, Available for Sale	1,727,381	4,375,620
Less Than 12 Months, Unrealized Losses, Available for Sale	(5,270)	(57,550)
12 Months or Longer, Fair Value, Available for Sale	0	0
12 Months or Longer, Unrealized Losses, Available for Sale	0	0
Total Available for sale securities, Fair Value	1,727,381	4,375,620
Total Available for sale securities, Unrealized Losses	(5,270)	(57,550)
FNMA pass-through certificates [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value, Held to Maturity	144
Less Than 12 Months, Unrealized Losses, Held to Maturity	(3)
12 Months or Longer, Fair Value, Held to Maturity	0
12 Months or Longer, Unrealized Losses, Held to Maturity	0
Total Held to maturity securities, Fair Value	144
Total Held to maturity securities, Unrealized Losses	(3)
FHLMC pass-through certificates [Member]
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Fair Value, Available for Sale	731,941	2,425,458
Less Than 12 Months, Unrealized Losses, Available for Sale	(1,896)	(32,116)
12 Months or Longer, Fair Value, Available for Sale	0	0
12 Months or Longer, Unrealized Losses, Available for Sale	0	0
Total Available for sale securities, Fair Value	731,941	2,425,458
Total Available for sale securities, Unrealized Losses	(1,896)	(32,116)
United States government-sponsored enterprise securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value, Held to Maturity	2,908,340	3,524,781
Less Than 12 Months, Unrealized Losses, Held to Maturity	(41,602)	(75,216)
12 Months or Longer, Fair Value, Held to Maturity	0	0
12 Months or Longer, Unrealized Losses, Held to Maturity	0	0
Total Held to maturity securities, Fair Value	2,908,340	3,524,781
Total Held to maturity securities, Unrealized Losses	(41,602)	(75,216)
FHLMC and FNMA - REMICs [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value, Held to Maturity		7,373
Less Than 12 Months, Unrealized Losses, Held to Maturity		(24)
12 Months or Longer, Fair Value, Held to Maturity		3,163
12 Months or Longer, Unrealized Losses, Held to Maturity		(8)
Total Held to maturity securities, Fair Value		10,536
Total Held to maturity securities, Unrealized Losses		$ (32)

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Emerging Issues Task Force (EITF)
-Number 03-1
-Paragraph 21
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name FASB Staff Position (FSP)
-Number FAS115-1/124-1
-Paragraph 17
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