Mortgage-Backed Securities (Schedule Of Mortgage-Backed Securities In A Loss Position) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 233,755	$ 2,827,798
Unrealized Loss - Less than 12 months	(138)	(33,125)
Fannie Mae Certificates ARMS [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Market Value - Less than 12 months		1,641,355
Unrealized Loss - Less than 12 months		(18,342)
Fannie Mae Certificates Fixed Rate [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Market Value - Less than 12 months	210,743	0
Unrealized Loss - Less than 12 months	(132)	0
Freddie Mac Certificates ARMS [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Market Value - Less than 12 months		1,186,443
Unrealized Loss - Less than 12 months		(14,783)
Freddie Mac Certificates Fixed Rate [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Market Value - Less than 12 months	23,012	0
Unrealized Loss - Less than 12 months	$ (6)	$ 0