Fair Value Measurements - Summary of Carrying Value and Estimated Fair Value of Long-Term Debt (Details) - Level 2 - Recurring basis - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Subordinated promissory note receivable, carrying value	$ 106.2	$ 95.3
Subordinated promissory note receivable, estimated fair value	111.0	83.3
Long-term debt excluding capital leases and credit facility, carrying value	2,282.3	2,280.6
Long-term debt excluding capital leases and credit facility, estimated fair value	$ 2,335.0	$ 2,439.8