Fair Value Disclosures - Narrative (Details) (USD $)	9 Months Ended				3 Months Ended		9 Months Ended		0 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Level 3	Dec. 31, 2011 Level 3	Sep. 30, 2012 Gains on Trading Assets	Sep. 30, 2011 Gains on Trading Assets	Sep. 30, 2012 Gains on Trading Assets	Sep. 30, 2011 Gains on Trading Assets	Sep. 30, 2012 Asset-backed Securities Floating Interest Rate [Member]	Dec. 31, 2011 Asset-backed Securities Floating Interest Rate [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3	Sep. 30, 2011 Fair Value, Measurements, Recurring [Member] Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Reset period of auction rate certificate interest rates	28 days
Derivative asset and liability credit risk valuation adjustment	$ 400,000	$ 200,000
Assets and Liabilities, Fair Value Disclosure			6,300,000,000	5,900,000,000
Assets and liabilities fair value as percent of total assets			50.00%	49.00%
Assets and liabilities fair value as percent of total investments			69.00%	72.00%
Trading Securities, Unrealized Holding Loss									3,130,000	3,342,000	400,000	2,000,000	2,000,000
Changes in fair value gain (losses), fair value option					$ (500,000)	$ (3,300,000)	$ (2,600,000)	$ (1,400,000)