Stock Repurchases (Details) - USD ($) shares in Millions		3 Months Ended	6 Months Ended
	Feb. 08, 2018	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Mar. 16, 2018
Stockholders' Equity Note [Abstract]
Stock repurchase program, authorized amount		$ 2,500,000,000.0	$ 2,500,000,000.0		$ 500,000,000
Stock repurchase program, remaining authorized amount		$ 546,000,000	$ 546,000,000
Accelerated share repurchases, initial price to be paid	$ 150,000,000
Accelerated share repurchases, percentage of shares to be repurchased	85.00%
Accelerated share repurchases, number of shares		0.7
Stock Repurchased During Period, Shares		1.0
Stock Repurchased During Period, Value		$ 200,200,000
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			$ 18,600,000	$ 14,100,000