Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013
ARS Purchase Commitments [Member] \| Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.62%
Current Yield	0.27%
ARS Purchase Commitments [Member] \| Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.65%
Current Yield	1.31%
ARS Purchase Commitments [Member] \| Other Member [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	64.60%
Auction Rate Securities [Member] \| Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.62%
Current Yield	0.27%
Auction Rate Securities [Member] \| Auction Rate Securities Other than Student Loan [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	85.50%
Auction Rate Securities [Member] \| Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.65%
Current Yield	1.31%
Auction Rate Securities [Member] \| Other Member [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	64.60%
Minimum [Member] \| ARS Purchase Commitments [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.38%
Current Yield	0.10%
Minimum [Member] \| Auction Rate Securities [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.38%
Current Yield	0.10%
Maximum [Member] \| ARS Purchase Commitments [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.88%
Current Yield	0.53%
Maximum [Member] \| Auction Rate Securities [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.88%
Current Yield	0.53%
Weighted Average [Member] \| ARS Purchase Commitments [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.65%
Duration	4 years
Current Yield	0.33%
Weighted Average [Member] \| ARS Purchase Commitments [Member] \| Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.62%
Duration	4 years 6 months
Current Yield	0.27%
Weighted Average [Member] \| ARS Purchase Commitments [Member] \| Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.65%
Duration	7 years
Current Yield	1.31%
Weighted Average [Member] \| ARS Purchase Commitments [Member] \| Other Member [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	64.60%
Weighted Average [Member] \| Auction Rate Securities [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.65%
Duration	4 years
Current Yield	0.33%
Weighted Average [Member] \| Auction Rate Securities [Member] \| Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.62%
Duration	4 years 6 months
Current Yield	0.27%
Weighted Average [Member] \| Auction Rate Securities [Member] \| Auction Rate Securities Other than Student Loan [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	85.50%
Weighted Average [Member] \| Auction Rate Securities [Member] \| Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.65%
Duration	7 years
Current Yield	1.31%
Weighted Average [Member] \| Auction Rate Securities [Member] \| Other Member [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	64.60%
Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 120,684
Valuation Adjustment	9,106
Fair Value	111,579
Level 3 [Member] \| Auction Rate Securities Owned [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	91,630
Valuation Adjustment	6,506
Fair Value	85,124
Level 3 [Member] \| Auction Rate Securities Owned [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	74,075
Valuation Adjustment	3,752
Fair Value	70,323
Valuation Technique	Discounted Cash Flow
Level 3 [Member] \| Auction Rate Securities Owned [Member] \| Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	8,230
Valuation Adjustment	813
Fair Value	7,417
Valuation Technique	Discounted Cash Flow
Level 3 [Member] \| Auction Rate Securities Owned [Member] \| Auction Rate Securities Other than Student Loan [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	5,975
Valuation Adjustment	866
Fair Value	5,109
Valuation Technique	Secondary Market Trading Activity
Level 3 [Member] \| Auction Rate Securities Owned [Member] \| Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	525
Valuation Adjustment	75
Fair Value	450
Valuation Technique	Discounted Cash Flow
Level 3 [Member] \| Auction Rate Securities Owned [Member] \| Auction Rate Securities Owned other [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	2,825
Valuation Adjustment	1,000
Fair Value	1,825
Valuation Technique	Secondary Market Trading Activity
Level 3 [Member] \| ARS Purchase Commitments [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	29,054
Valuation Adjustment	2,600
Fair Value	26,455
Level 3 [Member] \| ARS Purchase Commitments [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	10,746
Valuation Adjustment	535
Fair Value	10,211
Valuation Technique	Discounted Cash Flow
Level 3 [Member] \| ARS Purchase Commitments [Member] \| Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	16,221
Valuation Adjustment	1,603
Fair Value	14,619
Valuation Technique	Discounted Cash Flow
Level 3 [Member] \| ARS Purchase Commitments [Member] \| Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	1,304
Valuation Adjustment	185
Fair Value	1,119
Valuation Technique	Discounted Cash Flow
Level 3 [Member] \| ARS Purchase Commitments [Member] \| Auction Rate Securities Owned other [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	783
Valuation Adjustment	277
Fair Value	$ 506
Valuation Technique	Secondary Market Trading Activity