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Changes in Capital Structure - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions |  | 1 Months Ended | 3 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jan. 01, 2018 | Mar. 31, 2018 | Mar. 31, 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 |  |  |  |
Stock repurchase program, amount, beginning immediately |  |  |  | $ 500 |
Total number of shares purchased |  |  | 3,114,748 |  |
Commissions per share (in dollars per share) |  | $ 0.0001 |  |  |
Capital Allocation Plan |  |  |  |  |
Capital Structure |  |  |  |  |
Stock repurchase program, authorized amount |  |  |  | $ 1,000 |
Total number of shares purchased |  | 3,114,748 |  |  |
Average price paid per share (in dollars per share) |  | $ 29.75 |  |  |
Amounts paid for shares purchased |  | $ 93 |  |  |

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