Common Stock Incentive Plan - Performance Shares and Restricted Stock Units (Details) $ in Millions	6 Months Ended
Jun. 30, 2019 USD ($) shares
Performance Shares and Restricted Stock Units
Common Stock Incentive Plan
Unamortized share-based compensation expense \| $	$ 12.2
Performance Shares
Common Stock Incentive Plan
Shares granted in period (in shares)	115,137
Vesting rights percentage	50.00%
Vesting period (in years)	3 years
Restricted Stock Units
Common Stock Incentive Plan
Shares granted in period (in shares)	5,482
Vesting period (in years)	4 years

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

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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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Percentage of vesting of award under share-based payment arrangement.

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