Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Auction Rate Securities Owned
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Valuation Adjustment For Level Two and Three Assets	$ 4.1
Minimum [Member] | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	2.86%
Fair Value Unobservable Input Current Yield	2.69%
Maximum [Member] | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	3.89%
Maximum [Member] | ARS purchase commitments | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Unobservable Input Current Yield	4.05%
Other Assets | Minimum [Member] | ARS purchase commitments | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	2.86%
Fair Value Unobservable Input Current Yield	2.69%
Other Assets | Maximum [Member] | ARS purchase commitments | Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	3.89%
Fair Value Unobservable Input Current Yield	4.05%