Investment Securities (Schedule Of Exposure To Investment Security Issuers Exceed Ten Percentage Of Shareholders' Equity) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule of Investments [Line Items]
Amortized Cost	$ 1,765,597	$ 1,722,582
Three Investment Security Issuers Exceeding 10% Of Shareholders' Equity [Member]
Schedule of Investments [Line Items]
Amortized Cost	1,603,719
Market Value	1,636,439
Three Investment Security Issuers Exceeding 10% Of Shareholders' Equity [Member] | Federal National Mortgage Association (Fannie Mae) [Member]
Schedule of Investments [Line Items]
Amortized Cost	878,108
Market Value	896,142
Three Investment Security Issuers Exceeding 10% Of Shareholders' Equity [Member] | Government National Mortgage Association (Ginnie Mae) [Member]
Schedule of Investments [Line Items]
Amortized Cost	257,703
Market Value	263,122
Three Investment Security Issuers Exceeding 10% Of Shareholders' Equity [Member] | Federal Home Loan Mortgage Corporation (Freddie Mac) [Member]
Schedule of Investments [Line Items]
Amortized Cost	467,908
Market Value	$ 477,175