Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	5 years
Total stock-based compensation expense	$ 13,429,000	$ 7,413,000	$ 4,001,000
Tax benefit from stock-based compensation	7,581,000	12,719,000	5,048,000
Employee Stock \| 2015 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 100,000	$ 100,000	$ 100,000
Fair market value purchase discount (percent)	90.00%
Maximum number of shares participant may purchase as a percentage of base compensation (not to exceed) (percent)	10.00%
Maximum number of shares participant may purchase, fair market value (not to exceed) (shares)	$ 15,000
Purchase period (in years)	6 months

Share-based Compensation Arrangement by Share-based Payment Award, Maximum Fair Market Value of Shares Per Employee

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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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- Definition

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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- Definition

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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No definition available.

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- Definition

The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

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- Definition

Purchase price of common stock expressed as a percentage of its fair value.

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No definition available.

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