Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	3 Months Ended
	Mar. 31, 2017 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 118,735,000
Valuation Adjustment	(2,038,000)
Fair Value	$ 116,697,000
Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	1.71%
Municipal Auction Rate Securities 01
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	1.81%
Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	1.20%
U S Treasury Rate	2.21%
Auction Rate Securities Owned
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 92,300,000
Valuation Adjustment	(2,557,000)
Fair Value	89,743,000
Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	91,975,000
Valuation Adjustment	(2,530,000)
Fair Value	$ 89,445,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
Auction Rate Securities Owned | Municipal Auction Rate Securities 01
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 25,000
Valuation Adjustment	(1,000)
Fair Value	$ 24,000
Valuation Technique	Discounted Cash Flow
Discount Rate	3.17%
Duration	4 years 6 months
Fair Value Unobservable Input Current Yield	2.61%
Auction Rate Securities Owned | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 300,000
Valuation Adjustment	(26,000)
Fair Value	$ 274,000
Valuation Technique	Discounted Cash Flow
Discount Rate	3.41%
Duration	7 years
Fair Value Unobservable Input Current Yield	1.98%
Minimum [Member] | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	110.00%
Minimum [Member] | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.88%
Fair Value Unobservable Input Current Yield	1.40%
Maximum [Member] | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	150.00%
Maximum [Member] | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.56%
Maximum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Unobservable Input Current Yield	1.56%
Weighted Average | Auction Rate Securities Owned | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.21%
Duration	4 years
Fair Value Unobservable Input Current Yield	1.48%
Weighted Average | Auction Rate Securities Owned | Municipal Auction Rate Securities 01
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.17%
Duration	4 years 6 months
Fair Value Unobservable Input Current Yield	2.61%
Weighted Average | Auction Rate Securities Owned | Student Loan Auction Rate Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.41%
Duration	7 years
Fair Value Unobservable Input Current Yield	1.98%
Auction Rate Securities Purchase Commitment
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 26,435,000
Valuation Adjustment	(519,000)
Fair Value	$ 26,954,000
Other Assets | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Duration	4 years
Other Assets | Minimum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.88%
Fair Value Unobservable Input Current Yield	1.40%
Other Assets | Maximum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.56%
Fair Value Unobservable Input Current Yield	1.56%
Other Assets | Weighted Average | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.21%
Duration	4 years
Fair Value Unobservable Input Current Yield	1.48%
Other Assets | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 6,654,000
Valuation Adjustment	(878,000)
Fair Value	$ 7,532,000
Valuation Technique	Discounted Cash Flow
Accounts Payable and Other Liabilities [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Duration	4 years
Accounts Payable and Other Liabilities [Member] | Minimum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.88%
Fair Value Unobservable Input Current Yield	1.40%
Accounts Payable and Other Liabilities [Member] | Maximum [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.56%
Fair Value Unobservable Input Current Yield	1.56%
Accounts Payable and Other Liabilities [Member] | Weighted Average | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.21%
Duration	4 years
Fair Value Unobservable Input Current Yield	1.48%
Accounts Payable and Other Liabilities [Member] | Auction Rate Securities Purchase Commitment | Auction Rate Preferred Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 19,781,000
Valuation Adjustment	(359,000)
Fair Value	$ 19,422,000
Valuation Technique	Discounted Cash Flow