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Stock-Based Compensation - Additional Information (Q3) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense | $ 6.4 | $ 2.6 | $ 20.5 | $ 14.5 |  |  |  | $ 21.3 | $ 20.8 | $ 26.0
Unamortized compensation expense expected to be vested | 35.4 |  | 35.4 |  |  |  |  | 24.6 |  |
Stock Option |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense | 0.4 |  | 1.2 |  |  |  |  | 1.7 |  |
Restricted Stock |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense | 2.3 |  | 5.0 |  |  |  |  |  |  |
Restricted Stock Units (RSUs) |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense | 2.3 |  | 7.0 |  |  |  |  | 4.6 |  |
Performance Shares |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense | $ 1.2 |  | $ 6.4 |  |  |  |  | $ 6.5 |  |
Award vesting period (in years) |  |  | 3 years |  |  |  |  | 3 years |  |
Percentage of incentive stock awards expected to be vested and awarded of stocks granted (as a percent) |  |  |  |  |  |  |  | 84.00% |  |
Performance Shares | Minimum |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of incentive stock awards expected to be vested and awarded of stocks granted (as a percent) |  |  | 0.00% |  |  |  |  | 0.00% |  |
Performance Shares | Maximum |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of incentive stock awards expected to be vested and awarded of stocks granted (as a percent) |  |  | 200.00% |  |  |  |  | 200.00% |  |
Performance Shares | Scenario Forecast as of September 30, 2018 [Member] |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of incentive stock awards expected to be vested and awarded of stocks granted (as a percent) |  |  |  |  | 91.00% | 81.00% | 71.00% |  |  |
2011 Plan |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Number of shares available for grant (in shares) | 3,800,000 |  | 3,800,000 |  |  |  |  |  |  |
2011 Plan | Stock Option |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Award vesting period (in years) |  |  | 4 years |  |  |  |  | 4 years |  |
Plan term (in years) |  |  | 10 years |  |  |  |  | 10 years |  |
2011 Plan | Restricted Stock |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Award vesting period (in years) |  |  | 4 years |  |  |  |  | 4 years |  |
Directors Plan |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense | $ 0.2 |  | $ 0.9 |  |  |  |  |  |  |
Directors Plan | Restricted Stock |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Award vesting period (in years) |  |  | 1 year |  |  |  |  |  |  |
2000 Plan | Stock Option |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Award vesting period (in years) |  |  | 4 years |  |  |  |  | 4 years |  |
Plan term (in years) |  |  | 10 years |  |  |  |  | 10 years |  |
2000 Plan | Restricted Stock |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Award vesting period (in years) |  |  | 4 years |  |  |  |  | 4 years |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Percentage of vesting of share-based compensation awards. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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