Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014
Statement of Investments [Line Items]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	$ 30,424
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	30,517
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Amortized Cost Basis	67,544
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Fair Value	71,350
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis	199,377
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value	203,240
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	241,202
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	227,417
Available for Sale Securities, Debt Securities, Amortized Cost, Before Securities Without Debt Maturities	538,547
Available for Sale Securities, Debt Maturities, Before Securities Without Single Maturities	532,524
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	2,485,017
Available for Sale, Estimated Fair Value	2,455,464
Collateralized Mortgage Backed Securities [Member]
Statement of Investments [Line Items]
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	1,031,968
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	1,006,737
Collateralized Mortgage Obligations [Member]
Statement of Investments [Line Items]
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	914,502
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	$ 916,203