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Common Equity - Stock Options (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Options Activity |  |  |  |
Outstanding, shares, beginning balance | 4,452,533 | 4,644,214 |  |
Granted, shares |  | 710,710 | 552,215 | 794,764
Exercised, shares |  | (899,391) |  |
Forfeited, shares |  | (3,000) |  |
Outstanding, shares, ending balance |  | 4,452,533 | 4,644,214 |
Options - Weighted Average Exercise Price |  |  |  |
Outstanding, Weighted-Average Exercise Price, Beginning | $ 48.86 | $ 43.11 |  |
Granted, Weighted-Average Exercise Price |  | 65.59 |  |
Exercised, Weighted-Average Exercise Price |  | 32.39 |  |
Forfeited, Weighted-Average Exercise Price |  | 57.99 |  |
Outstanding, Weighted-Average Exercise Price, Ending |  | $ 48.86 | $ 43.11 |
Options - Additional Disclosures |  |  |  |
Outstanding, Weighted-Average Remaining Contractual Life (Years) |  | 6 years 1 month |  |
Outstanding, Aggregate Intrinsic Value |  | $ 90.8 |  |
Exercisable, shares |  | 2,838,609 |  |
Exercisable, Weighted-Average Exercise Price (in dollars per share) |  | $ 42.77 |  |
Exercisable, Weighted-Average Remaining Contractual Life (Years) |  | 4 years 11 months |  |
Exercisable, Aggregate Intrinsic Value |  | $ 75.2 |  |
Intrinsic value of options exercised |  | 32.4 | $ 33.8 | $ 55.4
Tax benefit from option exercises |  | 8.9 | $ 13.5 | $ 22.2
Compensation cost not yet recognized |  | $ 3.0 |  |
Weighted-average period over which unrecognized compensation cost is expected to be recognized |  | 1 year 8 months |  |
Estimated weighted-average fair value per stock option (in dollars per share) |  | $ 7.71 | $ 7.45 | $ 5.14
Subsequent event |  |  |  |
Options Activity |  |  |  |
Granted, shares | 476,418 |  |  |
Options - Weighted Average Exercise Price |  |  |  |
Granted, Weighted-Average Exercise Price | $ 68.18 |  |  |
Options - Additional Disclosures |  |  |  |
Estimated weighted-average fair value per stock option (in dollars per share) | $ 8.60 |  |  |

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested options awarded to employees as compensation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Amount of tax benefit from exercise of stock options granted under share-based compensation arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109197908&loc=SL79508275-113901 + 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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&loc=d3e5070-113901 + Details
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