Derivative Financial Instruments (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Cash Collateral Received For Derivative Instruments	1,210,000,000	1,150,000,000
Cash Collateral Provided For Derivative Instruments	1,070,000,000	1,480,000,000
Derivative Liability, Fair Value	412,000,000
Collateral already posted, aggregate fair value	9,000,000
Maximum additional amount of payments related to termination events	403,000,000
Fair Value Hedges [Member]
Derivative [Line Items]
Securities weighted-average life, years	7.3	7.4
Subordinated note matures	2018
Fair Value Hedges [Member] | 2.875% Senior Note [Member]
Derivative [Line Items]
Fixed interest rate	2.875%
Fair Value Hedges [Member] | 4.375% Senior Note [Member]
Derivative [Line Items]
Fixed interest rate	4.375%
Fair Value Hedges [Member] | 4.956% Subordinated Note [Member]
Derivative [Line Items]
Fixed interest rate	4.956%
Fair Value Hedges [Member] | 5.25% Subordinated Note [Member]
Derivative [Line Items]
Fixed interest rate	5.25%
Cash Flow Hedges [Member]
Derivative [Line Items]
Securities weighted-average life, years	2.6	2.8