Derivative Financial Instruments - Notional Amount of Interest Rate Swap Agreements Designated as Fair Value and Cash Flow Hedges (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Increase (decrease) in carrying value of long-term debt	$ (135)	$ 76
Fair Value Hedges
Derivative [Line Items]
Total	6,065	6,077
Fair Value Hedges | Investment securities available for sale
Derivative [Line Items]
Investment securities available for sale	2,565	2,577
Fair Value Hedges | Long-term debt
Derivative [Line Items]
Long-term debt	$ 3,500	$ 3,500