Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Nov. 08, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Public bonds	$ 2,892.5	$ 2,906.0	$ 2,300.0
Public bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Public bonds	2,316.6	2,650.8
Retail bond and private placement note | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value (excluding premium)	307.9	342.1
Reported Value Measurement | Public bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Public bonds	2,600.0	2,600.0
Reported Value Measurement | Retail bond and private placement note | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value (excluding premium)	$ 292.5	$ 306.0