Agency MBS Agency MBS (Unrealized loss positions) (Details) (Agency MBS [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012 Agreements	Dec. 31, 2011 Agreements
Agency MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized loss position, less than on year, Fair Value	$ 392,903	$ 680,101
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(4,281)	(6,765)
Mortgage Backed Securities, Number in unrealized loss position less than 12 months	42	54
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	323,732	160,544
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(4,832)	(1,684)
Mortgage Backed Securities, Unrealized Loss position for 12 months or more	35	27
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	716,635	840,645
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ (9,113)	$ (8,449)
Mortgage Backed Securities, Number of Securities in Continuous Unrealized Loss position	77	81