Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Nov. 08, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Public bonds	$ 2,924.9	$ 2,751.4	$ 2,300.0
Public bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Public bonds	3,031.1	2,618.4
Private placement note | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value (excluding premium)	177.5	168.4
Reported Value Measurement | Public bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Public bonds	2,760.0	2,600.0
Reported Value Measurement | Private placement note | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value (excluding premium)	$ 164.9	$ 151.4