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Changes in Capital Structure - Narrative (Details) - USD ($)				1 Months Ended		3 Months Ended			9 Months Ended
	Sep. 05, 2018	May 24, 2018	Jan. 01, 2018	Jul. 31, 2018	Jan. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Feb. 28, 2018	Dec. 31, 2017
Capital Structure
Common stock, shares authorized (in shares)						500,000,000			500,000,000		500,000,000
Shares issued under ESPP (in shares)			175,862		175,862
Stock repurchase program, authorized amount	$ 500,000,000	$ 354,000,000
Total number of shares purchased (in shares)									28,544,693
Commissions per share (in dollars per share)						$ 0.01			$ 0.01
Shares repurchased (in shares)	12,820,512,000,000						9,969,023
Treasury
Capital Structure
Shares issued under ESPP (in shares)			175,862
Total number of shares purchased (in shares)									28,544,693
Accelerated Share Repurchase Program
Capital Structure
Shares repurchased (in shares)				860,880
Value of shares received	$ 452,000,000	343,000,000
Average price paid per share under ASR Agreement (in dollars per share)				$ 32.69
Accelerated Share Repurchase Program | Additional Paid-in Capital
Capital Structure
Remaining amount recorded in additional paid in capital	48,000,000	$ 11,000,000
Accelerated Share Repurchase Program | Treasury
Capital Structure
Remaining amount recorded in additional paid in capital				$ 11,000,000
Capital Allocation Plan
Capital Structure
Stock repurchase program, authorized amount	$ 1,000,000,000					$ 1,000,000,000			$ 1,000,000,000	$ 1,000,000,000
Total number of shares purchased (in shares)						13,681,392	11,748,553	3,114,748	28,544,693
Amounts paid for shares purchased						$ 500,000,000	$ 407,000,000	$ 93,000,000	$ 1,000,000,000

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