Financial Instruments and Fair Value Measurements - Derivative Financial Instruments (Details) £ in Millions, $ in Millions	Sep. 30, 2020 GBP (£) derivative Rate	Sep. 30, 2020 USD ($) derivative Rate	Jun. 30, 2020 USD ($) derivative	Feb. 29, 2020 USD ($) derivative	Dec. 31, 2019 USD ($)
Derivative [Line Items]
Notional Amount		$ 1,223.3			$ 673.4
Fair Value - asset (liability)		(52.3)			(26.3)
Interest rate swap, maturity Sep 2021 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 6.8			7.0
Strike (percent)	6.03%	6.03%
Fair Value - asset (liability)		$ (0.2)			(0.2)
Interest rate swap, maturity Jun 2020 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 0.0			250.0
Strike (percent)	1.72%	1.72%
Fair Value - asset (liability)		$ 0.0			(0.1)
Interest rate swap, maturity Mar 2024 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 250.0			250.0
Strike (percent)	3.04%	3.04%
Fair Value - asset (liability)		$ (24.4)			(14.7)
Cross-currency swap, maturity May 2034 1 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		41.6			41.6
Fair Value - asset (liability)		$ 1.5			(2.6)
Cross-currency swap, maturity May 2034 1 | Designated as hedging instrument | Cash flow hedge | GBP | Minimum
Derivative [Line Items]
Strike (percent)	4.82%	4.82%
Cross-currency swap, maturity May 2034 1 | Designated as hedging instrument | Cash flow hedge | GBP | Maximum
Derivative [Line Items]
Strike (percent)	10.96%	10.96%
Cross-currency swap, maturity May 2034 1 | Designated as hedging instrument | Cash flow hedge | USD
Derivative [Line Items]
Strike (percent)	9.80%	9.80%
Cross-currency swap, maturity May 2034 2 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 41.6			41.6
Fair Value - asset (liability)		$ 1.5			(2.6)
Cross-currency swap, maturity May 2034 2 | Designated as hedging instrument | Cash flow hedge | GBP | Minimum
Derivative [Line Items]
Strike (percent)	4.82%	4.82%
Cross-currency swap, maturity May 2034 2 | Designated as hedging instrument | Cash flow hedge | GBP | Maximum
Derivative [Line Items]
Strike (percent)	10.96%	10.96%
Cross-currency swap, maturity May 2034 2 | Designated as hedging instrument | Cash flow hedge | USD
Derivative [Line Items]
Strike (percent)	9.803%	9.803%
Cross-currency swap, maturity May 2034 3 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 41.6			41.6
Fair Value - asset (liability)		$ 1.2			(2.9)
Cross-currency swap, maturity May 2034 3 | Designated as hedging instrument | Cash flow hedge | GBP | Minimum
Derivative [Line Items]
Strike (percent)	4.82%	4.82%
Cross-currency swap, maturity May 2034 3 | Designated as hedging instrument | Cash flow hedge | GBP | Maximum
Derivative [Line Items]
Strike (percent)	10.96%	10.96%
Cross-currency swap, maturity May 2034 3 | Designated as hedging instrument | Cash flow hedge | USD
Derivative [Line Items]
Strike (percent)	9.745%	9.745%
Cross-currency swap, maturity May 2034 4 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 41.6			41.6
Fair Value - asset (liability)		$ 0.9			(3.2)
Cross-currency swap, maturity May 2034 4 | Designated as hedging instrument | Cash flow hedge | GBP | Minimum
Derivative [Line Items]
Strike (percent)	4.82%	4.82%
Cross-currency swap, maturity May 2034 4 | Designated as hedging instrument | Cash flow hedge | GBP | Maximum
Derivative [Line Items]
Strike (percent)	10.96%	10.96%
Cross-currency swap, maturity May 2034 4 | Designated as hedging instrument | Cash flow hedge | USD
Derivative [Line Items]
Strike (percent)	9.755%	9.755%
Currency exchange swap | Not designated as hedging instrument
Derivative [Line Items]
Notional Amount	£ 224.9	$ 300.1			0.0
Strike (rate) | Rate	133.00%	133.00%
Fair Value - asset (liability)		$ 9.5			0.0
Forward-starting swap, maturity Jun 2033 1 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 75.0			0.0
Strike (percent)	2.02%	2.02%
Fair Value - asset (liability)		$ (7.1)			0.0
Forward-starting swap, maturity Nov 2032 1 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 75.0			0.0
Strike (percent)	1.94%	1.94%
Fair Value - asset (liability)		$ (7.1)			0.0
Forward-starting swap, maturity Nov 2032 2 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 25.0			0.0
Strike (percent)	1.67%	1.67%
Fair Value - asset (liability)		$ (1.7)			0.0
Forward-starting swap, maturity Jun 2033 2 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 125.0			0.0
Strike (percent)	1.75%	1.75%
Fair Value - asset (liability)		$ (8.5)			0.0
Forward-starting swap, hybrid debt, maturity Nov 2032 3 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 125.0			0.0
Strike (percent)	1.88%	1.88%
Fair Value - asset (liability)		$ (11.0)			0.0
Forward-starting swap, hybrid debt, maturity Jun 2033 3 | Designated as hedging instrument | Cash flow hedge
Derivative [Line Items]
Notional Amount		$ 75.0			0.0
Strike (percent)	2.00%	2.00%
Fair Value - asset (liability)		$ (6.9)			$ 0.0
Treasury rate locks | Cash flow hedge
Derivative [Line Items]
Notional Amount				$ 500.0
Number of derivative instruments | derivative				5
Forward starting swaps | Cash flow hedge
Derivative [Line Items]
Notional Amount			$ 500.0
Number of derivative instruments | derivative	6	6	6