Stock-based Compensation (Details) - Equity Incentive Plan - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Stock-based Compensation
Recorded compensation costs for the RSAs and RSUs	$ 2.7	$ 0.7
Restricted Stock Awards (RSAs)
Stock-based Compensation
Weighted average grant date fair value (in dollars per share)	$ 52.83	$ 35.40
Restricted Stock Awards (RSAs) \| Officers and Directors
Stock-based Compensation
Awards granted (in shares)	36,183	46,135
Awards canceled (in shares)	8,334	9,464
Restricted Stock Awards (RSAs) \| Director
Stock-based Compensation
Vesting period	12 months
Restricted Stock Awards (RSAs) \| Officer
Stock-based Compensation
Vesting period	36 months
Restricted Stock Unit Award (RSUs) \| Officer
Stock-based Compensation
Awards granted (in shares)	26,473	28,594
Awards canceled (in shares)	31,177	24,009
Weighted average grant date fair value (in dollars per share)	$ 52.83	$ 35.40
Awards issued (in shares)	62,726	48,753
Expiration period of award	3 years

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.

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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 number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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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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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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