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Capital Structure - Common Stock (Details) - USD ($) $ / shares in Units, $ in Millions |  |  |  |  | 3 Months Ended | 12 Months Ended | 15 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
Feb. 15, 2018 | Feb. 01, 2018 | Jan. 19, 2018 | Jan. 01, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2015
Employee Stock Purchase Plan and 2013 Capital Allocation Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends paid per share (in usd per share) |  |  |  |  |  | $ 0.030 | $ 0.030 | $ 0.030 | $ 0.030 | $ 0.030 | $ 0.030 | $ 0.030 | $ 0.145 | $ 0.145 | $ 0.145 | $ 0.145 | $ 0.145 |  |  |  | $ 0.145 |
Common stock dividends, proposed annual percentage decrease |  |  |  |  |  |  |  |  |  | 79.00% |  |  |  |  |  |  |  |  |  | 79.00% |  |
Dividends Per Common Share (in usd per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.12 | $ 0.24 | 0.58 |
Eligible compensation (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.00% |  |  |
Exercise price as a percentage of fair value (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 85.00% |  |  |
Treasury stock reserved for issuance under the ESPP (in shares) |  |  |  |  |  | 3,107,050 |  |  |  |  |  |  |  |  |  |  |  |  | 3,107,050 |  |  |
Commissions per share (in usd per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.015
Scenario, Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee Stock Purchase Plan and 2013 Capital Allocation Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, dividends, proposed annual amount, per share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0012 | $ 0.58 |
Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee Stock Purchase Plan and 2013 Capital Allocation Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable, Date Declared |  |  |  | Jan. 19, 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Per Common Share (in usd per share) |  |  |  | $ 0.00030 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends payable, date to be paid |  | Feb. 15, 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends payable, date of record |  |  | Feb. 01, 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued to employee from treasury stock (in shares) |  |  |  |  | 175,862 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event | Scenario, Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee Stock Purchase Plan and 2013 Capital Allocation Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, dividends, proposed annual amount, per share |  | $ 0.0012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term incentive plans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee Stock Purchase Plan and 2013 Capital Allocation Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Capital Shares Reserved for Future Issuance |  |  |  |  |  | 19,597,433 |  |  |  |  |  |  |  |  |  |  |  |  | 19,597,433 |  |  |
Capital Allocation Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Employee Stock Purchase Plan and 2013 Capital Allocation Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase program, authorized amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 481 |  |  |  |  |  |  | $ 481 | $ 481
Total number of shares purchased (in shares) |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,558,920 | 11,104,184 | 4,379,907 | 3,146,484 | 1,624,360 |  |  |  | 25,813,855
Average price paid per share (in usd per share) | [1] |  |  |  |  |  |  |  |  |  |  |  |  | $ 15.03 | $ 15.06 | $ 24.53 | $ 25.15 | $ 26.95 |  |  |  |
Amounts paid for shares purchased |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 84 | $ 167 | $ 107 | $ 79 | $ 44 |  |  |  | $ 481
[1] | The average price paid per share and amounts paid for shares purchased exclude the commissions of $0.015 per share paid in connection with the share repurchase.

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- Definition Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644 + Details
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