Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 124,523,000
Valuation Adjustment	2,492,000
Fair Value	$ 122,031,000
OMHHF
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Securities owned		$ 3,600,000
Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
U S Treasury Rate	1.78%
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	2.17%
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	$ 109,000,000
Valuation Adjustment	2,216,000
Fair Value	106,784,000
Auction Rate Securities Owned [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	108,675,000
Valuation Adjustment	2,198,000
Fair Value	$ 106,477,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
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	2,000
Fair Value	$ 23,000
Valuation Technique	Secondary Market Trading Activity
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	16,000
Fair Value	$ 284,000
Valuation Technique	Discounted Cash Flow
Discount Rate	3.37%
Duration	7 years
Current Yield	2.52%
Auction Rate Securities Owned [Member] | Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.96%
Current Yield	1.56%
Auction Rate Securities Owned [Member] | Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.67%
Current Yield	1.93%
Auction Rate Securities Owned [Member] | Maximum [Member] | Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs Observable Trades In Inactive Market For In Portfolio Securities	90.25%
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	2.25%
Duration	4 years
Current Yield	1.71%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Municipal Auction Rate Securities 01 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs Observable Trades In Inactive Market For In Portfolio Securities	90.25%
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	3.37%
Duration	7 years
Current Yield	2.52%
Auction Rate Securities Purchase Commitment [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 15,523,000
Valuation Adjustment	276,000
Fair Value	15,247,000
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	15,496,000
Valuation Adjustment	275,000
Fair Value	$ 15,221,000
Valuation Technique	Discounted Cash Flow
Duration	4 years
Auction Rate Securities Purchase Commitment [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.37%
Duration	7 years
Current Yield	2.52%
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	1.96%
Current Yield	1.56%
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	2.67%
Current Yield	1.93%
Auction Rate Securities Purchase Commitment [Member] | Maximum [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs Observable Trades In Inactive Market For In Portfolio Securities	90.25%
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	2.25%
Duration	4 years
Current Yield	1.71%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs Observable Trades In Inactive Market For In Portfolio Securities	90.25%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.37%
Duration	7 years
Current Yield	2.52%
Auction Rate Securities Purchase Commitment [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 2,000
Valuation Adjustment	0
Fair Value	$ 2,000
Valuation Technique	Secondary Market Trading Activity
Auction Rate Securities Purchase Commitment [Member] | Student Loan Auction Rate Securities [Member]
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