Fair Value Measurements - Transfers between Level 1 and Level 2 (Details) $ in Millions	Dec. 31, 2018 USD ($) stock	Dec. 31, 2017 USD ($) stock	Dec. 31, 2016 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	2	4	6
Fair value of assets transferred from Level 2 to Level 1 | $	$ 5	$ 23	$ 35
Number of stocks transferred from Level 1 to Level 2 | stock	2	2	7
Fair value of assets transferred from Level 1 to Level 2 | $	$ 6	$ 11	$ 28
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	0	0	3
Fair value of assets transferred from Level 2 to Level 1 | $	$ 0	$ 0	$ 0
Number of stocks transferred from Level 1 to Level 2 | stock	0	1	2
Fair value of assets transferred from Level 1 to Level 2 | $	$ 0	$ 0	$ 0