Stock-based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted-average exercise price per share:
Compensation expense recorded during the year	$ 25,251	$ 20,697	$ 16,505
Stock Options
Stock-Based Compensation
Percentage of the award vesting at each of the three anniversaries	33.00%
Contractual term of the stock options	10 years
Fair Value Assumptions
Expected volatility (as a percent)	30.00%	31.00%	28.00%
Expected dividends (as a percent)	0.00%	0.00%	0.00%
Risk-free rate, Minimum (as a percent)	1.70%	0.80%	0.30%
Risk-free rate, Maximum (as a percent)	3.00%	2.70%	2.10%
Summary of equity awards, options
Balance at beginning of period (in units)	3,457	2,738	1,845
Granted (in units)	674	861	962
Exercised (in units)	(322)	(90)	(9)
Forfeited or canceled (in units)	(116)	(52)	(60)
Balance at end of period (in units)	3,693	3,457	2,738
Weighted-average exercise price per share:
Outstanding at beginning of year (in dollars per share)	$ 16.40	$ 15.81	$ 18.17
Granted (in dollars per share)	24.40	18.05	11.29
Exercised (in dollars per share)	16.24	15.04	17.33
Forfeited	18.46	15.58	15.66
Outstanding at end of year (in dollars per share)	$ 17.81	$ 16.40	$ 15.81
Compensation expense recorded during the year	$ 6,218	$ 4,909	$ 4,464
Number of options exercisable at end of year	2,235
Weighted average exercise price per exercisable option	$ 16.69
Weighted-average remaining contractual term:
Outstanding at end of year	7 years
Exercisable at end of year	6 years
Aggregate intrinsic value
Outstanding at end of year	$ 14,764
Exercisable at end of year	$ 10,207
Number of shares expected to vest	1,368
Weighted-average vesting period (in months)	10 months
Minimum \| Stock Options
Fair Value Assumptions
Expected grantee forfeiture rate (as a percent)	0.00%	0.00%	0.00%
Maximum \| Stock Options
Fair Value Assumptions
Expected grantee forfeiture rate (as a percent)	(23.20%)	(21.10%)	(20.20%)

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Value of weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Percentage of vesting of share-based compensation awards.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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