Derivative Financial Instruments - Narrative (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) security	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Cash collateral received for derivative instruments	$ 1,450	$ 1,790
Cash collateral provided for derivative instruments	2,500	4,790
Fair value of derivative liabilities	10,609	$ 15,234
Collateral Already Posted, Aggregate Fair Value	$ 14
Securities weighted average life (in years)	5 years 4 months 24 days	5 years 10 months 18 days
Credit swap agreements
Derivative [Line Items]
Fair value of derivative liabilities	$ 1,410
Maximum additional amount of payments related to termination events	$ 1,390
Interest rate swap | Fair Value Hedges | 5.38% senior note due 2017
Derivative [Line Items]
Fixed interest rate	5.38%
Interest rate swap | Fair Value Hedges | 1.35% senior note due 2018
Derivative [Line Items]
Fixed interest rate	1.35%
Interest rate swap | Fair Value Hedges | 2.55% note due 2020
Derivative [Line Items]
Fixed interest rate	2.55%
Interest rate swap | Fair Value Hedges | 4.38% note due 2021
Derivative [Line Items]
Fixed interest rate	4.38%
Interest rate swap | Fair Value Hedges | 3.70% senior note due 2023
Derivative [Line Items]
Fixed interest rate	3.70%
Interest rate swap | Fair Value Hedges | 3.30% senior note due 2024
Derivative [Line Items]
Fixed interest rate	3.30%
Interest rate swap | Fair Value Hedges | 3.55% Notes Due 2025
Derivative [Line Items]
Fixed interest rate	3.55%
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	7
Interest rate swap | Fair Value Hedges | Subordinated note
Derivative [Line Items]
Number of securities | security	2