Securities (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012
Held to Maturity:
Due in one year or less, estimated fair market value
Due after one year through five years, estimated fair market value	2,475
Due after five years through ten years, estimated fair market value	8,420
Due after ten years, estimated fair market value	4,142,704
Total held to maturity, estimated fair market value	4,153,599
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Due after one through five years, fair value	311,208
Due after ten years, estimated fair market value	9,039,762
Total available for sale, estimated fair market value	9,350,970
Mortgage-backed securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Due after one through five years, amortized cost
Due after ten years, amortized cost	8,868,311
Total available for sale, amortized cost	8,868,311
Investment Securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Due after one through five years, amortized cost	307,620
Due after ten years, amortized cost
Total available for sale, amortized cost	307,620
Mortgage-backed securities [Member]
Held to Maturity:
Due in one year or less, amortized cost
Due after one year through five years, amortized cost	2,344
Due after five years through ten years, amortized cost	7,876
Due after ten years, amortized cost	3,843,513
Total held to maturity, amortized cost	3,853,733
Investment Securities [Member]
Held to Maturity:
Due in one year or less, amortized cost
Due after one year through five years, amortized cost
Due after five years through ten years, amortized cost
Due after ten years, amortized cost	39,011
Total held to maturity, amortized cost	$ 39,011