Stock Based Compensation - Restricted Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Unrecognized compensation cost related to unvested stock options	$ 11.9		$ 11.9
Stock based compensation expense	1.3	$ 1.3	3.5	$ 2.6
RSU
Unrecognized compensation cost related to unvested stock options	7.0		$ 7.0
Vesting period			3 years 2 months 12 days
Stock based compensation expense	$ 0.5	$ 0.3	$ 0.9	$ 0.5
Number of Shares
Beginning Balance			133
Granted (in shares)			245
Settled (in shares)			(30)
Forfeited (in shares)			(1)
Ending balance	347		347
Weighted Average Grant Date Fair Value
Beginning Balance			$ 28.85
Granted (per share)			19.49
Settled (per share)			27.32
Forfeited (per share)			32.53
Ending balance	$ 22.38		$ 22.38

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

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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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 non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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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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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Weighted average remaining contractual term for fully vested and expected to vest exercisable or convertible options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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