Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	$ 48,079
Valuation Adjustment For Level Three Asset and Liability	3,811
Fair Value Of Level Three Assets and Liabilities	44,268
Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	40,650
Valuation Adjustment	2,697
Fair Value	37,953
Auction Rate Preferred Securities 01 [Member] \| Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	18,950
Valuation Adjustment	2,697
Fair Value	$ 16,253
Fair Value Measurement, Valuation Techniques	Tender Offer (7)
Municipal Auction Rate Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	2.49%
Municipal Auction Rate Securities [Member] \| Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	$ 75
Valuation Adjustment	0
Fair Value	$ 75
Fair Value Measurement, Valuation Techniques	Discounted Cash Flow
Fair Value Input, Discount Rate	3.96%
Fair Value Unobservable Inputs Duration	2 years
Fair Value Unobservable Input Current Yield	5.50%
Municipal Auction Rate Securities [Member] \| Weighted Average [Member] \| Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	3.96%
Fair Value Unobservable Inputs Duration	2 years
Fair Value Unobservable Input Current Yield	5.50%
Student Loan Auction Rate Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	1.20%
U S Treasury Rate	2.48%
Student Loan Auction Rate Securities \| Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	$ 275
Valuation Adjustment	0
Fair Value	$ 275
Fair Value Measurement, Valuation Techniques	Discounted Cash Flow
Fair Value Input, Discount Rate	3.42%
Fair Value Unobservable Inputs Duration	4 years
Fair Value Unobservable Input Current Yield	3.95%
Student Loan Auction Rate Securities \| Weighted Average [Member] \| Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	3.42%
Fair Value Unobservable Inputs Duration	4 years
Fair Value Unobservable Input Current Yield	3.95%
Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
U S Treasury Rate	2.60%
Auction Rate Preferred Securities \| Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	$ 21,350
Valuation Adjustment	0
Fair Value	$ 21,350
Fair Value Measurement, Valuation Techniques	Discounted Cash Flow
Fair Value Unobservable Inputs Duration	1 year
Auction Rate Preferred Securities \| Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	110.00%
Auction Rate Preferred Securities \| Minimum [Member] \| Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	2.63%
Fair Value Unobservable Input Current Yield	2.72%
Auction Rate Preferred Securities \| Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Spread To U S Treasury Rate	150.00%
Auction Rate Preferred Securities \| Maximum [Member] \| Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	3.58%
Fair Value Unobservable Input Current Yield	4.09%
Auction Rate Preferred Securities \| Weighted Average [Member] \| Auction rate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input, Discount Rate	3.57%
Fair Value Unobservable Inputs Duration	1 year
Fair Value Unobservable Input Current Yield	4.07%
Auction Rate Securities Purchase Commitment
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	$ 7,429
Valuation Adjustment For Level Three Liabilities	1,114
Fair Value Of Level Three Liabilities	6,315
Auction Rate Securities Purchase Commitment \| Auction Rate Preferred Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Level Three Principal Amount	7,429
Valuation Adjustment For Level Three Liabilities	1,114
Fair Value Of Level Three Liabilities	$ 6,315
Fair Value Measurement, Valuation Techniques	Tender Offer (7)

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Fair Value Measurement, Valuation Techniques

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Fair value of the level 3 assets used for the quantitative disclosure.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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