Variable Interest Entities (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Variable Interest Entities [Abstract]
Investment securities related to state and political subdivisions	$ 2,330,000,000		$ 2,330,000,000
Variable interest entity, other short-term borrowings	1,930,000,000		1,950,000,000
Weighted average life of trusts (in years)	6 years 3 months 15 days	6 years 6 months
Total standby bond-purchase agreement committed to trusts	1,960,000,000
Letters of credit amounts outstanding	684,000,000
Standby purchase agreements and letters of credit commitments utilized	0
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	24,000,000		18,000,000
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	60,000,000		50,000,000
Liabilities	$ 10,000,000