Derivative Financial Instruments - Narrative (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) security	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Cash collateral received for derivative instruments	$ 1,520	$ 1,790
Cash collateral provided for derivative instruments	2,710	4,790
Fair value of derivative liabilities	10,329	$ 15,234
Collateral Already Posted, Aggregate Fair Value	$ 19
Securities weighted average life (in years)	5 years 5 months 38 days	5 years 10 months 18 days
Fair Value Hedges | 1.35% senior note due 2018
Derivative [Line Items]
Fixed interest rate	1.35%
Fair Value Hedges | 4.38% note due 2021
Derivative [Line Items]
Fixed interest rate	4.38%
Fair Value Hedges | 3.70% senior note due 2023
Derivative [Line Items]
Fixed interest rate	3.70%
Fair Value Hedges | 3.30% senior note due 2024
Derivative [Line Items]
Fixed interest rate	3.30%
Fair Value Hedges | 5.38% subordinated note due 2017
Derivative [Line Items]
Fixed interest rate	5.38%
Credit swap agreements
Derivative [Line Items]
Fair value of derivative liabilities	$ 1,090
Maximum additional amount of payments related to termination events	$ 1,070
Interest rate swap | Fair Value Hedges | 4.96% subordinated note due 2018
Derivative [Line Items]
Fixed interest rate	4.956%
Interest rate swap | Fair Value Hedges | 3.10% subordinated note due 2023
Derivative [Line Items]
Fixed interest rate	3.10%
Interest rate swap | Fair Value Hedges | Senior notes
Derivative [Line Items]
Number of securities | security	5
Interest rate swap | Fair Value Hedges | Subordinated note
Derivative [Line Items]
Number of securities | security	2