Performance Based Plan Unvested Rollforward (Details) - Performance Shares - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Performance Share Rollforward [Line Items]
Unvested Performance Awards Outstanding	207,712	227,697
Above target performance adjustment	122,281
Vested in 2020	(244,562)
Granted in 2020	102,296
Weighted Average Grant Date Fair Value (Performance Awards)	$ 33.58	$ 27.50	$ 25.32
Weighted Average Grant Date Fair Value of Above Target Performance Adjustment	25.37
Weighted Average Grant Date Fair Value of Vested Performance Shares	25.37
Weighted Average Grant Date Fair Value of Performance Awards Granted During the Period	$ 37.29	$ 29.98	$ 25.37
Performance Award Compensation Expense Recognized	$ 7,800	$ 6,200	$ 5,700
Performance Award unrecognized compensation expense	$ 913
Performance Award unrecognized compensation expense weighted average recognition period	1 year 6 months

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

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for 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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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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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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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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Other than shares newly issued, the number of additional shares issued (for example, a stock split) or canceled (for example, to correct a share issuance), during the period under the plan.

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Weighted average price at which grantees could acquire or could have acquired the underlying shares with respect to any other type of change in shares reserved for issuance.

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