Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Detail) (Level 3 [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 120,554
Valuation Adjustment	8,711
Fair Value	111,843
Auction Rate Securities Owned [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	91,530
Valuation Adjustment	6,505
Fair Value	85,025
Auction Rate Securities Owned [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	73,325
Valuation Adjustment	3,895
Fair Value	69,430
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Owned [Member] \| Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.64%
Duration	4 years 6 months
Current Yield	0.25%
Principal	8,180
Valuation Adjustment	817
Fair Value	7,363
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Owned [Member] \| Municipal Auction Rate Securities 02 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	92.60%
Principal	5,975
Valuation Adjustment	441
Fair Value	5,534
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]
Discount Rate	3.50%
Duration	7 years
Current Yield	0.84%
Principal	425
Valuation Adjustment	69
Fair Value	356
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Owned [Member] \| Other Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	64.60%
Principal	3,625
Valuation Adjustment	1,283
Fair Value	2,342
Valuation Technique	Secondary Market Trading Activity
Auction Rate Securities Owned [Member] \| Minimum [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.42%
Duration	4 years
Current Yield	0.17%
Auction Rate Securities Owned [Member] \| Maximum [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.94%
Current Yield	0.43%
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	1.70%
Duration	4 years
Current Yield	0.31%
Auction Rate Securities Owned [Member] \| Weighted Average [Member] \| Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.64%
Duration	4 years 6 months
Current Yield	0.25%
Auction Rate Securities Owned [Member] \| Weighted Average [Member] \| Municipal Auction Rate Securities 02 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Observable trades in inactive market for in portfolio securities	92.60%
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	3.50%
Duration	7 years
Current Yield	0.84%
Auction Rate Securities Owned [Member] \| Weighted Average [Member] \| Other Auction Rate Securities [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 Commitments to Purchase [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	29,024
Valuation Adjustment	2,206
Fair Value	26,818
Auction Rate Securities Commitments to Purchase [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Duration	4 years
Principal	14,946
Valuation Adjustment	763
Fair Value	14,183
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Commitments to Purchase [Member] \| Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.50%
Current Yield	0.84%
Principal	567
Valuation Adjustment	92
Fair Value	475
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Commitments to Purchase [Member] \| Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.64%
Duration	4 years 6 months
Current Yield	0.25%
Principal	13,511
Valuation Adjustment	1,351
Fair Value	$ 12,160
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Commitments to Purchase [Member] \| Minimum [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.42%
Current Yield	0.17%
Auction Rate Securities Commitments to Purchase [Member] \| Minimum [Member] \| Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Duration	7 years
Auction Rate Securities Commitments to Purchase [Member] \| Maximum [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.94%
Current Yield	0.43%
Auction Rate Securities Commitments to Purchase [Member] \| Weighted Average [Member] \| Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.70%
Duration	4 years
Current Yield	0.31%
Auction Rate Securities Commitments to Purchase [Member] \| Weighted Average [Member] \| Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	3.50%
Duration	7 years
Current Yield	0.84%
Auction Rate Securities Commitments to Purchase [Member] \| Weighted Average [Member] \| Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.64%
Duration	4 years 6 months
Current Yield	0.25%