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Debt (Narrative - Corporate Activities) (Details) - Corporate Activities [Member] - Convertible Notes [Member] - USD ($)	1 Months Ended	9 Months Ended	12 Months Ended
	Aug. 31, 2016	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019
4.00% Convertible Notes Due 2024 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 115,000,000.0	$ 115,000,000.0
Interest rate, stated percentage			4.00%	4.00%	4.00%
Debt Instrument, Maturity Date		Jul. 01, 2024
Common stock for conversion, shares			64.1540
Debt Instrument Convertible Conversion Price Benchmark1			$ 1,000
Debt conversion price			$ 15.59
Debt Conversion, Sale Price Of Common Stock Percent, Minimum			140.00%
Debt Instrument, Redemption Price, Percentage			100.00%
4.125% Convertible Notes Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 170,000,000.0	$ 170,000,000.0	$ 170,000,000.0
Interest rate, stated percentage	4.125%	4.125%
Debt conversion, principal amounts for integral multiples	$ 1,000
Common stock for conversion, shares	35.7143
Debt conversion price	$ 28.00
Conversion price percentage	140.00%
Principal amount of notes, percentage	100.00%	100.00%

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