Fair Value Measurements Fair Value Measurements - Carrying Amount and Fair Value of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Publicly traded debt | Carrying Amount
Debt Instrument [Line Items]
Debt fair value	$ 8,203.2	$ 8,731.7	$ 8,742.7
Publicly traded debt | Fair Value
Debt Instrument [Line Items]
Debt fair value	8,735.8	8,330.2	9,054.3
Non-traded debt | Carrying Amount
Debt Instrument [Line Items]
Debt fair value	277.3	283.6	1,144.2
Non-traded debt | Fair Value
Debt Instrument [Line Items]
Debt fair value	$ 270.7	$ 272.7	$ 1,088.6