Agency MBS Unrealized Loss Position (Details) (Agency MBS [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012 Agreements	Dec. 31, 2011 Agreements
Agency MBS [Member]
Schedule of Agency MBS [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,026,277	$ 680,101
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(6,552)	(6,765)
Mortgage Backed Securities, Number in unrealized loss position less than 12 months	83	54
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	271,719	160,544
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(5,797)	(1,684)
Mortgage Backed Securities, Number in Unrealized Loss position for 12 months or more	34	27
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,297,996	840,645
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ (12,349)	$ (8,449)
Mortgage Backed Securities, Number of Securities in Continuous Unrealized Loss position	117	81