Fair Value Disclosures - Narrative (Details) (USD $)			0 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2013 Level 3	Dec. 31, 2012 Level 3	Jun. 30, 2013 Asset-backed Securities [Member] Floating Interest Rate [Member]	Dec. 31, 2012 Asset-backed Securities [Member] Floating Interest Rate [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Level 3	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Level 3		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets and Liabilities, Fair Value Disclosure	$ 6,800,000,000	$ 7,100,000,000
Assets and liabilities fair value as percent of total assets	52.00%	56.00%
Assets and liabilities fair value as percent of total investments	73.00%	73.00%
Trading Securities, Unrealized Holding Loss			2,762,000	3,080,000	300,000	2,400,000	100,000	[1]	1,600,000	[2]
Trading Securities, Unrealized Holding Gain			$ 0	$ 0

[1] Includes unrealized losses of $0.1 million attributable to assets still held as of June 30, 2013 that are recorded in "Losses on trading assets.

[2] Includes unrealized losses of $1.6 million attributable to assets still held as of June 30, 2012 that are recorded in "Losses on trading assets.