Subsequent Events (Dividends - Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended													12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Estimated aggregate dividend payment														$ 1,717	$ 1,736	$ 1,653
Series A Preferred Stock [Member]
Subsequent Event [Line Items]
Preferred stock, dividends		$ 6	$ 6	$ 7	$ 6	$ 6	$ 6	$ 7	$ 6	$ 6	$ 6	$ 7	$ 6	25	25	25
Series B Preferred Stock [Member]
Subsequent Event [Line Items]
Preferred stock, dividends		0	0	0	0	0	0	0	0	0	0	24	24	0	0	48
Common stock
Subsequent Event [Line Items]
Estimated aggregate dividend payment		$ 422	$ 427	$ 431	$ 437	$ 441	$ 441	$ 441	$ 413	$ 419	$ 420	$ 420	$ 394	$ 1,717	$ 1,736	$ 1,653
Subsequent Event [Member]
Subsequent Event [Line Items]
Estimated aggregate dividend payment	$ 416
Subsequent Event [Member] | Dividends Declared [Member] | Series A Preferred Stock [Member]
Subsequent Event [Line Items]
Preferred stock, dividend rate	$ 0.250
Preferred stock, dividends	$ 6
Subsequent Event [Member] | Dividends Declared [Member] | Common stock
Subsequent Event [Line Items]
Approved annual dividend, amount per share	$ 0.400