Stock Options - Assumptions Used to Calculate Fair Value of Options (Details) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted average assumptions
Dividend yield	0.00%
Options
Weighted average assumptions
Dividend yield	0.00%	0.00%	0.00%
Weighted average fair value (in dollars per share)	$ 16.20	$ 0.40	$ 0.47
Options \| Minimum
Weighted average assumptions
Expected life	5 years 6 months	5 years 9 months	6 years 3 months
Expected volatility	37.50%	37.50%	40.60%
Risk-free interest rate	2.38%	1.88%	1.27%
Options \| Maximum
Weighted average assumptions
Expected life	6 years 3 months	6 years 3 months	6 years 3 months
Expected volatility	49.80%	41.50%	40.60%
Risk-free interest rate	3.07%	2.32%	2.25%

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Price of a single share of a number of saleable stocks of a company.

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No definition available.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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