Stock-Based Compensation - Options Valuation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Stock-Based Compensation
Total stock-based compensation expense (in dollars)	$ 7,173	$ 6,906	$ 14,310	$ 13,758
Stock options
Stock-Based Compensation
Total stock-based compensation expense (in dollars)	$ 2,364	$ 2,642	$ 4,871	$ 5,316
Valuation assumptions:
Expected volatility (as a percent)			56.10%	69.60%
Expected term (years)			5 years 3 months	6 years 7 days
Risk-free interest rate (as a percent)			1.22%	2.50%
Weighted average grant-date fair value (in dollars per share)			$ 33.78	$ 34.94

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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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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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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