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Employee Stock Purchase Plan, Stock Options and Warrants - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |  |  |  |
Total intrinsic value of stock options exercised | $ 9,264 | $ 3,648 | $ 7,548 |
Cash received from stock option exercises | 5,552 | 4,577 | 6,227 |
Excess tax benefit from the exercise of stock options | $ 2,264 | $ 669 | $ 2,124 |
Number of Shares |  |  |  |
Beginning balance (in shares) | 2,817 |  |  |
Granted (in shares) | 1,297 |  |  |
Exercised (in shares) | (404) |  |  |
Forfeited/expired (in shares) | (87) |  |  |
Outstanding at December 31 (in shares) | 3,623 | 2,817 |  |
Exercisable (in shares) |  |  |  | 1,110
Ending vested and expected to vest (in shares) |  |  |  | 3,484
Weighted Average Exercise Price |  |  |  |
Beginning balance (in dollars per share) | $ 20.40 | $ 15.32 |  | $ 20.40
Granted (in dollars per share) | 29.31 |  |  |
Exercised (in dollars per share) | 14.02 |  |  |
Forfeited/expired (in dollars per share) | 18.79 |  |  |
Outstanding at December 31 (in dollars per share) | $ 20.40 | $ 15.32 |  |
Exercisable (in dollars per share) |  |  |  | 14.35
Ending vested and expected to vest (in dollars per share) |  |  |  | $ 20.23
Outstanding, remaining contractual term | 4 years 6 months 26 days |  |  |
Exercisable, remaining contractual term | 2 years 6 months 18 days |  |  |
Ending vested and expected to vest, remaining contractual term | 4 years 6 months 7 days |  |  |
Outstanding, intrinsic value |  |  |  | $ 82,615
Exercisable, intrinsic value |  |  |  | 32,019
Ending vested and expected to vest, intrinsic value |  |  |  | $ 80,052

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- Definition Aggregate proceeds received by the entity during the annual period from exercises of stock or unit options and conversion of similar instruments granted under equity-based payment arrangements. + 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 (j) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period. + 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 (j) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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