Non Agency MBS Non-Agency MBS, Unrealized loss positions (Details) (Mortgage-backed Securities, Issued by Private Enterprises [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012 Agreements	Dec. 31, 2011 Agreements
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized loss position, less than on year, Fair Value	$ 4,351	$ 153,974
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(120)	(5,075)
Mortgage Backed Securities, Number in unrealized loss position less than 12 months	3	6
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	11,700	2,993
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(1,504)	(379)
Mortgage Backed Securities, Unrealized Loss position for 12 months or more	9	7
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	16,051	156,967
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ (1,624)	$ (5,454)
Mortgage Backed Securities, Number of Securities in Continuous Unrealized Loss position	12	13