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Stock-Based Compensation - Summary of Stock Options Outstanding (Details) - $ / shares shares in Millions | 12 Months Ended |  |  |
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding By Exercise Price [Line Items] |  |  |  |
Options Outstanding, Shares | 2.3 | 2.6 | 2.1 | 2.0
Options Outstanding, Weighted Average Remaining life | 6 years 2 months 12 days |  |  |
Options Outstanding, Weighted Average Exercise Price (usd per share) | $ 11.08 | $ 10.71 | $ 10.29 | $ 9.28
Options Exercisable, Shares | 1.6 | 1.6 | 1.4 |
Options Exercisable, Weighted Average Exercise Price (usd per share) | $ 9.57 | $ 8.96 | $ 8.47 |
$5.01 — $10.00 |  |  |  |
Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding By Exercise Price [Line Items] |  |  |  |
Range lower (usd per share) | 5.01 |  |  |
Range upper (usd per share) | $ 10.00 |  |  |
Options Outstanding, Shares | 1.0 |  |  |
Options Outstanding, Weighted Average Remaining life | 4 years |  |  |
Options Outstanding, Weighted Average Exercise Price (usd per share) | $ 6.63 |  |  |
Options Exercisable, Shares | 1.0 |  |  |
Options Exercisable, Weighted Average Exercise Price (usd per share) | $ 6.63 |  |  |
10.01 — 15.00 |  |  |  |
Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding By Exercise Price [Line Items] |  |  |  |
Range lower (usd per share) | 10.01 |  |  |
Range upper (usd per share) | $ 15.00 |  |  |
Options Outstanding, Shares | 0.8 |  |  |
Options Outstanding, Weighted Average Remaining life | 7 years 6 months |  |  |
Options Outstanding, Weighted Average Exercise Price (usd per share) | $ 13.02 |  |  |
Options Exercisable, Shares | 0.4 |  |  |
Options Exercisable, Weighted Average Exercise Price (usd per share) | $ 13.30 |  |  |
15.01 — 20.00 |  |  |  |
Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding By Exercise Price [Line Items] |  |  |  |
Range lower (usd per share) | 15.01 |  |  |
Range upper (usd per share) | $ 20.00 |  |  |
Options Outstanding, Shares | 0.5 |  |  |
Options Outstanding, Weighted Average Remaining life | 8 years 1 month 6 days |  |  |
Options Outstanding, Weighted Average Exercise Price (usd per share) | $ 16.51 |  |  |
Options Exercisable, Shares | 0.2 |  |  |
Options Exercisable, Weighted Average Exercise Price (usd per share) | $ 16.09 |  |  |

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- Definition Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding By Exercise Price [Line Items] + References No definition available. + Details
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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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