Common stock Common Stock (ESPP B-S-M option pricing model assumptions) (Details) - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ESPP plan details [Abstract]
Stock Shares Issued During Period, ESPP	155,373	157,426
Two Thousand Twelve Employee Stcok Purchase Plan [Member] \| Employee Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%
ESPP plan details [Abstract]
ESPP-Maximum Annual Purchase by Employee, Percent	15.00%
Share-based Compensation Arrangements by Share-based Payment Award, Offering Period, Subsequent Offers	6 months
ESPP-Discount of market Price	85.00%
Share-based Comp, Per Share Wtd Avg Price of Shares Purchased	$ 22.35	$ 20.77
Common stock (in shares) reserved for ESPP	873,011
Two Thousand Twelve Employee Stcok Purchase Plan [Member] \| Employee Stock [Member] \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (years)	6 months	6 months	6 months
Expected Volatility Rate	33.00%	33.00%	29.00%
Risk-free interest rate	1.59%	2.09%	0.61%
Two Thousand Twelve Employee Stcok Purchase Plan [Member] \| Employee Stock [Member] \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (years)	6 months	6 months	6 months
Expected Volatility Rate	50.00%	37.80%	32.00%
Risk-free interest rate	2.51%	2.51%	1.39%

Aggregate number of common shares reserved for future issuance.

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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 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 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 weighted average of per share prices paid for shares purchased on the open market for issuance to employees under the plan.

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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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Number of shares issued during the period as a result of an employee stock purchase plan.

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ESPP plan details [Abstract]

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Share-based Compensation Arrangements by Share-based Payment Award, Discount of the lower of the fair market value of the Company's common stock on the first trading day of the offering period or on the last day of the offering period.

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