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Investment Securities - Narrative (Details) (USD $)
3 Months Ended 3 Months Ended 6 Months Ended
Jun. 30, 2012
position
Dec. 31, 2011
position
Dec. 31, 2011
A Minus Rating
position
Jun. 30, 2012
BBB Plus Rating
position
Dec. 31, 2011
BBB Plus Rating
position
Jun. 30, 2011
Investment Securities
Jun. 30, 2012
Investment Securities
Jun. 30, 2011
Investment Securities
Jun. 30, 2012
Floating Interest Rate [Member]
Asset-backed Securities
Dec. 31, 2011
Floating Interest Rate [Member]
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]                    
Proceeds from sale of available-for-sale investment securities $ 0         $ 75,100,000 $ 5,028,000 $ 153,609,000    
Number of securities in unrealized loss positions 36 44 1 1 1          
Trading securities weighted-average yield                 4.33% 4.36%
Available-for-sale Securities, Gross Realized Gains           $ 38,000 $ 28,000 $ 200,000