Fair Value Measurements - Schedule of Fixed Rate Long-term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Public bonds	$ 2,898.0	$ 2,906.0
Public bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Public bonds	2,525.9	2,650.8
Retail Bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retail bond and private placement note	326.7	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 Bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Retail bond and private placement note	$ 298.0	$ 306.0