Stock Repurchases (Details) - USD ($) shares in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Mar. 16, 2018
Stock repurchase program, authorized amount	$ 2,500,000,000			$ 2,500,000,000		$ 500,000,000
Stock repurchase program, remaining authorized amount	$ 46,000,000			$ 46,000,000
Stock Repurchased During Period, Shares	0.5	0.6	0.4
Stock Repurchased During Period, Value	$ 150,000,000.0	$ 150,000,000.0	$ 100,000,000.0
Shares repurchased from employees who surrendered shares to satisfy minimum tax withholding obligations				0.1
Shares repurchased from employees who surrendered shares to satisfy minimum tax withholding obligations, value				$ 16,900,000	$ 21,100,000