Fair Value Disclosures - Narrative (Details) (USD $)	3 Months Ended	12 Months Ended			0 Months Ended		3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Level 3	Dec. 31, 2012 Level 3	Mar. 31, 2013 Asset-backed Securities [Member] Floating Interest Rate [Member]	Dec. 31, 2012 Asset-backed Securities [Member] Floating Interest Rate [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Level 3	Mar. 31, 2012 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	$ 700,000	$ 600,000
Lower of cost or fair value adjustment on loans held for sale		(5,900,000)
Assets and Liabilities, Fair Value Disclosure			6,800,000,000	7,100,000,000
Assets and liabilities fair value as percent of total assets			53.00%	56.00%
Assets and liabilities fair value as percent of total investments			74.00%	73.00%
Trading Securities, Unrealized Holding Loss					2,884,000	3,080,000	100,000
Trading Securities, Unrealized Holding Gain					$ 0	$ 0		$ 800,000