Investments (Available-for-sale Investments) (Details Textual) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Cash and cash equivalents	$ 559		$ 559		$ 509
Total realized gains	5	$ 34	22	$ 65
Total realized losses	$ 120	$ 87	$ 271	$ 171