Fair Value Measurements - Transfers between Level 1 and Level 2 (Details) $ in Millions	Dec. 31, 2016 USD ($) stock	Dec. 31, 2015 USD ($) stock	Dec. 31, 2014 USD ($) stock
Perpetual preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 2 to Level 1 | stock	6	5	14
Fair value of assets transferred from Level 2 to Level 1 | $	$ 35	$ 19	$ 96
Number of stocks transferred from Level 1 to Level 2 | stock	7	7	13
Fair value of assets transferred from Level 1 to Level 2 | $	$ 28	$ 31	$ 83
Common stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 2 to Level 1 | stock	3	7	0
Fair value of assets transferred from Level 2 to Level 1 | $	$ 0	$ 80	$ 0
Number of stocks transferred from Level 1 to Level 2 | stock	2	0	7
Fair value of assets transferred from Level 1 to Level 2 | $	$ 0	$ 0	$ 26
Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 2 to Level 1 | stock	0	2	1
Fair value of assets transferred from Level 2 to Level 1 | $	$ 0	$ 11	$ 5
Number of stocks transferred from Level 1 to Level 2 | stock	0	0	0
Fair value of assets transferred from Level 1 to Level 2 | $	$ 0	$ 0	$ 0