Held-to-Maturity (HTM) Securities (Interest Rate Payment Terms) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 6,335,836	$ 8,832,845
Held To Maturity Securities Other Than Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	989,439	2,317,293
MBS [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,346,397	6,515,552
Fixed Interest Rate [Member] | Held To Maturity Securities Other Than Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	812,864	2,136,243
Fixed Interest Rate [Member] | MBS [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,890,057	2,437,697
Adjustable Interest Rate [Member] | Held To Maturity Securities Other Than Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	176,575	181,050
Adjustable Interest Rate [Member] | MBS [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,456,340	$ 4,077,855