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Earnings Per Share (Tables) | 6 Months Ended
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Jun. 30, 2018
Earnings Per Share [Abstract] |
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] | The following table sets forth the computation of basic and diluted earnings per share on net income (in thousands, except per share information):
|  | Three months ended June 30, |  | Six months ended June 30,
|  | 2018 |  | 2017 |  | 2018 |  | 2017
Earnings: |  |  |  |  |  |  |  |
Net income (numerator for basic and diluted calculations) |  | $ | 204,374 |  |  | $ | 232,190 |  |  | $ | 304,604 |  |  | $ | 377,702 |
Shares: |  |  |  |  |  |  |  |
Weighted average outstanding shares (denominator for basic calculation) |  | 64,071 |  |  | 64,449 |  |  | 64,278 |  |  | 64,401 |
Equivalent shares from outstanding stock options |  | 1,179 |  |  | 1,159 |  |  | 1,277 |  |  | 1,204 |
Denominator for diluted calculation |  | 65,250 |  |  | 65,608 |  |  | 65,555 |  |  | 65,605 |
Earnings per share: |  |  |  |  |  |  |  |
Basic |  | $ | 3.19 |  |  | $ | 3.60 |  |  | $ | 4.74 |  |  | $ | 5.86 |
Diluted |  | $ | 3.13 |  |  | $ | 3.54 |  |  | $ | 4.65 |  |  | $ | 5.76 |
Schedule Of Earnings Per Share Basic And Diluted By Common Class [Text Block] | The following table presents approximate amounts of stock options and performance contingent shares excluded from the calculation of common equivalent shares (in millions):
|  | Three months ended June 30, |  | Six months ended June 30,
|  | 2018 |  | 2017 |  | 2018 |  | 2017
Excluded from common equivalent shares: |  |  |  |  |  |  |  |
Stock options |  | 0.2 |  |  | 0.2 |  |  | 0.3 |  |  | 0.3 |
Performance contingent shares |  | 0.2 |  |  | 0.3 |  |  | 0.2 |  |  | 0.3 |

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- Definition Tabular disclosure of securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) in the future that were not included in the computation of diluted EPS because to do so would increase EPS amounts or decrease loss per share amounts for the period presented, by antidilutive securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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- Definition Tabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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