Off-Balance Sheet Guarantees and Long Term Standby Purchase Commitments - Long Term Standby Purchase Commitments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Long-Term Standby Purchase Commitments [Member]
Long-term Purchase Commitment [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 2,200,000,000	$ 2,300,000,000
Weighted average remaining maturity, Loans underlying guarantees not held by transferor	14 years 3 months 18 days	13 years 10 months 24 days
Guarantee obligations issued after January 1, 2003 [Member]
Long-term Purchase Commitment [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	3,800,000,000	3,900,000,000
Guarantee obligations issued after January 1, 2003 [Member] | Long-Term Standby Purchase Commitments [Member]
Long-term Purchase Commitment [Line Items]
Guarantor Obligations, Current Carrying Value	$ 26,800,000	$ 26,300,000