Non-Agency MBS Unrealized loss position (Details) (Mortgage-backed Securities, Issued by Private Enterprises [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012 Agreements	Dec. 31, 2011 Agreements
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 13,877	$ 153,974
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(76)	(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	2,701	2,993
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(198)	(379)
Mortgage Backed Securities, Number in Unrealized Loss position for 12 months or more	8	7
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	16,578	156,967
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ (274)	$ (5,454)
Mortgage Backed Securities, Number of Securities in Continuous Unrealized Loss position	11	13