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Equity Compensation Plans - Restricted Stock (Details Textual) - USD ($) $ / shares in Units, shares in Millions, $ 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]
Weighted-Average Grant Date Fair Value, Awards granted	$ 40.86	$ 41.69	$ 45.57
Market value of restricted stock shares vesting	$ 32.8	$ 17.0	$ 22.7
Restricted stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-Average Grant Date Fair Value, Awards granted	$ 40.86
Market-based Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	4 years
Performance based awards granted to executive management and certain key employees shares	0.1
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	2 years
Minimum \| Restricted stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	2 years
Maximum \| Restricted stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	4 years