Stockholders' Equity Effect of VE Territory Covered Losses Recovery on the Company Repurchasing its Common Stock (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended	12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018
Conversion of Stock [Line Items]
Recovery Through Conversion Rate Adjustment		$ 0
U.K.& I preferred stock
Conversion of Stock [Line Items]
Effective price per share (in USD per share)	[1]	$ 137.19	$ 113.05
Recovery Through Conversion Rate Adjustment		$ 5	$ 35
Europe preferred stock
Conversion of Stock [Line Items]
Effective price per share (in USD per share)	[1]	$ 137.19	$ 112.92
Recovery Through Conversion Rate Adjustment		$ 1	$ 21

[1] Effective price per share for the quarter is calculated using the volume-weighted average price of the Company’s class A common stock over a pricing period in accordance with the Company’s current certificates of designations for its series B and C convertible participating preferred stock. Effective price per share for the fiscal year is calculated using the weighted-average effective prices of the respective adjustments made during the year.