Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	6 Months Ended
	Jun. 30, 2016 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 117,567,000
Valuation Adjustment	830,000
Fair Value	$ 116,737,000
Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	0.86%
Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	0.94%
Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	1.20%
U S Treasury Rate	1.29%
Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	110.00%
Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Spread To U S Treasury Rate	150.00%
Auction Rate Securities Owned [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 90,700,000
Valuation Adjustment	1,599,000
Fair Value	89,101,000
Auction Rate Securities Owned [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	86,750,000
Valuation Adjustment	1,156,000
Fair Value	$ 85,594,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
Current Yield	0.77%
Auction Rate Securities Owned [Member] | Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 25,000
Valuation Adjustment	0
Fair Value	$ 25,000
Valuation Technique	Discounted Cash Flow
Discount Rate	1.64%
Duration	4 years 6 months
Current Yield	1.19%
Auction Rate Securities Owned [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 300,000
Valuation Adjustment	21,000
Fair Value	$ 279,000
Valuation Technique	Discounted Cash Flow
Discount Rate	2.49%
Duration	7 years
Current Yield	1.43%
Auction Rate Securities Owned [Member] | Other Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 3,625,000
Valuation Adjustment	422,000
Fair Value	$ 3,203,000
Valuation Technique	Secondary Market Trading Activity
Percent of Par	88.36%
Auction Rate Securities Owned [Member] | Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	0.94%
Auction Rate Securities Owned [Member] | Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.29%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.11%
Duration	4 years
Current Yield	0.77%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.64%
Duration	4 years 6 months
Current Yield	1.19%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.49%
Duration	7 years
Current Yield	1.43%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Other Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Percent of Par	88.36%
Auction Rate Securities Purchase Commitment [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 26,867,000
Valuation Adjustment	(769,000)
Fair Value	27,636,000
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	6,372,000
Valuation Adjustment	(911,000)
Fair Value	$ 7,283,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
Current Yield	0.77%
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 20,495,000
Valuation Adjustment	142,000
Fair Value	$ 20,353,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
Current Yield	0.77%
Auction Rate Securities Purchase Commitment [Member] | Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	0.94%
Auction Rate Securities Purchase Commitment [Member] | Minimum [Member] | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	0.94%
Auction Rate Securities Purchase Commitment [Member] | Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.29%
Auction Rate Securities Purchase Commitment [Member] | Maximum [Member] | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.29%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.11%
Duration	4 years
Current Yield	0.77%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Auction Rate Preferred Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.11%
Duration	4 years
Current Yield	0.77%