Fair Value Measurements (Details 1) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014 stock	Dec. 31, 2013 stock	Dec. 31, 2012 stock
Perpetual preferred stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 2 to Level 1	14	15	2
Fair value of assets transferred from Level 2 to Level 1	$ 96	$ 70	$ 16
Number of stocks transferred from Level 1 to Level 2	13	2	7
Fair value of assets transferred from Level 1 to Level 2	83	10	41
Common stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 2 to Level 1	0	2	0
Fair value of assets transferred from Level 2 to Level 1	0	35	0
Number of stocks transferred from Level 1 to Level 2	7	0	0
Fair value of assets transferred from Level 1 to Level 2	26	0	0
Redeemable preferred stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 2 to Level 1	1	3	1
Fair value of assets transferred from Level 2 to Level 1	5	21	2
Number of stocks transferred from Level 1 to Level 2	0	0	0
Fair value of assets transferred from Level 1 to Level 2	$ 0	$ 0	$ 0