Investment Securities (Details 2) (USD $) In Thousands	Sep. 30, 2011
Debt securities available for sale:
Due in one year or less	$ 23,908
Due after one year through five years	77,765
Due after five years through ten years	15,648
Due after ten years	258,851
Total available for available for sale (amortized cost)	376,172
Mortgage-backed securities available for sale	5,169,651
Total	5,545,823
Debt securities available for sale:
Due in one year or less	23,994
Due after one year through five years	79,896
Due after five years through ten years	16,947
Due after ten years	229,190
Total available for available for sale (fair value)	350,027
Mortgage-backed securities available for sale	5,108,945
Total	5,458,972
Debt securities held to maturity:
Due in one year or less	23,957
Due after one year through five years	21,814
Due after five years through ten years	134,730
Due after ten years	15,995
Total available for held to maturity (at cost)	196,496
Mortgage-backed securities held to maturity	965,341
Total	1,161,837
Debt securities held to maturity:
Due in one year or less	24,144
Due after one year through five years	22,731
Due after five years through ten years	140,377
Due after ten years	16,167
Total available for held to maturity (fair value)	203,419
Mortgage-backed securities held to maturity	893,796
Total	$ 1,097,215