Fair Value Measurements - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Parenthetical) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents and Short-Term Investments
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Cash	$ 328.1	$ 248.0
Cash and Cash Equivalents and Short-Term Investments | Fair Value Measurements on a Recurring Basis
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Cash	328.1	248.0
Restricted Cash and Cash Equivalents and Restricted Investments
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Restricted cash	58.0	50.2
Restricted Cash and Cash Equivalents and Restricted Investments | Fair Value Measurements on a Recurring Basis
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Restricted cash	58.0	50.2
Prepaid Expenses and Other Current Assets
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Restricted cash	1.9	1.4
Prepaid Expenses and Other Current Assets | Fair Value Measurements on a Recurring Basis
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Restricted cash	$ 1.9	$ 1.4