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Share-Based Compensation - Performance Restricted Stock Units - Narrative (Details) - PRSUs shares in Thousands, $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2020 shares	Dec. 31, 2019 shares	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2021 USD ($) metric shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2020 metric shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of performance award metrics | metric							4
Number of units increased (decreased) during the period (in shares) | shares	(125)	1			(2)
Recognized share-based compensation expense (benefit) | $			$ 0.3	$ 0.2	$ 0.7	$ 0.8
Unrecognized compensation cost | $			$ 1.6		$ 1.6
Nonvested shares outstanding (in shares) | shares	333	260	305	329	305	329	333
Weighted-average period over which unrecognized compensation cost is expected to be recognized (years)					2 years 2 months 12 days
Executive and Senior Officers
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period (years)					1 year
Requisite service period					5 years
Executive and Senior Officers | Anniversary 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage (ratably on each of the first four anniversary date)					25.00%
Executive and Senior Officers | Anniversary 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage (ratably on each of the first four anniversary date)					25.00%
Executive and Senior Officers | Anniversary 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage (ratably on each of the first four anniversary date)					25.00%
Executive and Senior Officers | Anniversary 4
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage (ratably on each of the first four anniversary date)					25.00%
Executive Officers
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period (years)					1 year
Number of performance award metrics | metric					3
Average share price trading period					20 days		20 days
Last days trading period					20 days		20 days
Dividend yield					0.00%

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

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