Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2017
Subsequent Event [Line Items]
Treasury Stock, Shares, Acquired		21,405,327	16,038,791
Treasury Stock, Value, Acquired, Cost Method		$ 1,041	$ 858
Estimated aggregate dividend payment		$ 416	$ 437
Subsequent Event [Member]
Subsequent Event [Line Items]
Treasury Stock, Shares, Acquired	7,630,398
Treasury Stock, Value, Acquired, Cost Method	$ 350
Approved dividend, amount per share	$ 0.42
Estimated aggregate dividend payment	$ 428