Common Stock Repurchase Program - Additional Information (Details) shares in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2018 USD ($) shares
Equity [Abstract]
Common stock repurchased, share repurchase program	$ 250,000,000	$ 1,130,000,000	$ 1,250,000,000
Common stock repurchased, share repurchase program, share | shares	3.8	20.4	23.0
Share repurchase program, amount	$ 1,500,000,000	$ 1,500,000,000	$ 1,500,000,000