Available-for-Sale (AFS) Securities (Redemption Terms) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Amortized Cost:
Amortized Cost Basis	$ 6,297,647		$ 4,518,370
Fair Value:
Fair Value	6,326,162		4,356,147
Total Non-MBS [Member]
Amortized Cost:
Due in one year or less	326,992		1,993
Due after one year through five years	1,049,913		900,000
Due in more than ten years	33,995		0
Amortized Cost Basis	1,410,900	[1]	901,993
Fair Value:
Due in one year or less	326,773		1,998
Due after one year through five years	1,049,809		900,574
Due in more than ten years	33,992		0
Fair Value	1,410,574		902,572
MBS [Member]
Amortized Cost:
Amortized Cost Basis	4,886,747		3,616,377
Fair Value:
Fair Value	$ 4,915,588		$ 3,453,575

[1] Amortized cost includes adjustments made to the cost basis of an investment for accretion and/or amortization, collection of cash, and/or previous OTTI recognized in earnings.