Shareholders' Equity (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shareholders' Equity Disclosure [Line Items]
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries	$ 3,700	$ 3,900
Cash and cash equivalents	11,981	10,430	$ 10,923	$ 7,495
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	20,700
Statutory Accounting Practices, Statutory Capital and Surplus Required	$ 12,200
Stock Repurchase Program, Number of Shares Authorized to be Repurchased	100
Extraordinary Dividends [Member]
Shareholders' Equity Disclosure [Line Items]
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries	$ 1,100	$ 3,300