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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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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The estimated measure of the maximum 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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The estimated measure of the minimum 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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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the expected term of a nonvested share or option award issued to other than an employee.

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Period an equity-based award is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The estimated measure of the percentage amount by which a share price is expected to fluctuate during the expected term of a nonvested share or option award issued to other than an employee.

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The risk-free interest rate assumption that is used in valuing a nonvested share or option award issued to other than an employee.

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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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Stock Option Plan - Stock Option Assumptions (Details)	3 Months Ended		12 Months Ended
Stock Option Plan - Stock Option Assumptions (Details)	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, maximum			38.80%	41.00%
Common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months
Expected volatility	42.70%	38.10%
Expected volatility, minimum			38.10%	39.00%
Risk-free interest rate	2.54%	2.68%
Risk-free interest rate, minimum			2.68%	1.03%
Risk-free interest rate, maximum			2.98%	2.25%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Nonemployee options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonemployee dividend yield			0.00%	0.00%
Minimum \| Common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)				1 year
Minimum \| Nonemployee options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonemployee expected term (in years)			5 years	3 years 9 months
Nonemployee expected volatility			38.00%	39.00%
Nonemployee risk free interest rate			2.71%	2.06%
Maximum \| Common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)				6 years 3 months
Maximum \| Nonemployee options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonemployee expected term (in years)			6 years 3 months	9 years 9 months
Nonemployee expected volatility			38.80%	56.00%
Nonemployee risk free interest rate			2.90%	2.39%