Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Statement of Investments [Line Items]
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	36,715
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	36,777
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	0
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Fair Value	0
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Amortized Cost Basis	64,996
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Fair Value	69,357
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	0
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Fair Value	0
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis	179,933
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value	190,269
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	288,895
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	269,692
Held to Maturity Debt Securities, Before Securities Without Single Maturity	0
Held to Maturity, Debt Securities Fair Value, Before Securities Without Single Maturity	0
Available for Sale Securities, Debt Securities, Amortized Cost, Before Securities Without Debt Maturities	570,539
Available for Sale Securities, Debt Maturities, Before Securities Without Single Maturities	566,095
Held to Maturity Securities, Amortized Cost	257
Held-to-maturity Securities, Debt Maturities, Fair Value	279
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	2,653,332
Available for Sale, Estimated Fair Value	2,687,957
Collateralized Mortgage Backed Securities [Member]
Statement of Investments [Line Items]
Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Net Carrying Amount	257
Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Fair Value	279
Collateralized Mortgage Obligations [Member]
Statement of Investments [Line Items]
Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Net Carrying Amount	0
Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Fair Value	0
Collateralized Mortgage Backed Securities [Member]
Statement of Investments [Line Items]
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	789,048
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	816,063
Collateralized Mortgage Obligations [Member]
Statement of Investments [Line Items]
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	1,293,745
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	$ 1,305,799