Investment Securities - Narrative (Details) (USD $)	3 Months Ended					3 Months Ended	9 Months Ended
	Sep. 30, 2012 position	Dec. 31, 2011 position	Sep. 30, 2012 A Minus Rating position	Dec. 31, 2011 A Minus Rating position	Dec. 31, 2011 External Credit Rating, BBB Plus Rating [Member] position	Sep. 30, 2011 Investment Securities	Sep. 30, 2012 Investment Securities	Sep. 30, 2011 Investment Securities	Sep. 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]
Number of securities in Loss Position	13	44	1	1	2
Proceeds from sale of available-for-sale investment securities	$ 0					$ 294,700,000	$ 5,028,000	$ 447,864,000
Trading securities weighted-average yield									4.30%	4.36%
Available-for-sale Securities, Gross Realized Gains						84,000	28,000	279,000
Available-for-sale Securities, Gross Realized Losses						$ (10,000)		$ (10,000)