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 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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Stock-based Compensation (Details) - Equity Incentive Plan - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Stock-based Compensation
Recorded compensation costs for the RSAs and RSUs			$ 1.5	$ 1.5
Restricted Stock Awards (RSAs)
Stock-based Compensation
Weighted average grant date fair value (in dollars per share)			$ 35.40	$ 36.75
Restricted Stock Awards (RSAs) \| Officers and Directors
Stock-based Compensation
Awards granted (in shares)	0	0	46,135	48,717
Awards canceled (in shares)	3,842	3,284	13,306	14,186
Restricted Stock Awards (RSAs) \| Employees
Stock-based Compensation
Vesting period			36 months
Restricted Stock Awards (RSAs) \| Director
Stock-based Compensation
Vesting period			12 months
Restricted Stock Unit Award (RSUs) \| Officer
Stock-based Compensation
Awards granted (in shares)			28,594	31,389
Awards canceled (in shares)			24,009	19,735
Weighted average grant date fair value (in dollars per share)			$ 35.40	$ 36.75
Awards issued (in shares)			48,753	38,709
Expiration period of award			3 years