Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Sep. 30, 2019 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Valuation Adjustment For Level Three Asset and Liability	$ 5.1
Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Valuation Adjustment For Level Three Asset and Liability	4.1
Auction Rate Securities Purchase Commitment
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Valuation Adjustment For Level Three Asset and Liability	$ 1.0