Employee Incentive Plans - Summary of Non-Cash Compensation Expense under LTI Programs (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total non-cash compensation expense	$ 9.6	$ 9.7	$ 10.1
Restricted Stock And Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total non-cash compensation expense	6.1	6.1	5.8
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total non-cash compensation expense	$ 3.5	$ 3.6	$ 4.3

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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