Available-for-Sale Securities Summary of Available-for-Sale Securities by Contractual Maturity (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Amortized Cost
Less than one year	$ 8,554
Greater than one year but less than five years	3,167
Greater than five years but less than ten years	37
Greater than ten years	11
Amortized Cost	11,769	9,628
Fair Value
Less than one year	8,554
Greater than one year but less than five years	3,172
Greater than five years but less than ten years	37
Greater than ten years	11
Total	$ 11,774	$ 9,625