Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 119,593
Valuation Adjustment	8,346
Fair Value	111,247
Auction Rate Securities Purchase Commitment [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	20,213
Valuation Adjustment	1,514
Fair Value	18,699
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	9,033
Valuation Adjustment	443
Fair Value	8,590
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Purchase Commitment [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	527
Valuation Adjustment	54
Fair Value	473
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Purchase Commitment [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	10,653
Valuation Adjustment	1,017
Fair Value	9,636
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Owned [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	99,380
Valuation Adjustment	6,832
Fair Value	92,548
Auction Rate Securities Owned [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	78,850
Valuation Adjustment	4,059
Fair Value	74,791
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Owned [Member] | Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	10,030
Valuation Adjustment	958
Fair Value	9,072
Valuation Technique	Discounted Cash Flow
Discount Rate	2.51%
Duration	4 years 6 months
Current Yield	0.24%
Auction Rate Securities Owned [Member] | Municipal Auction Rate Securities 02 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	5,975
Valuation Adjustment	441
Fair Value	5,534
Valuation Technique	Secondary Market Trading Activity
Observable trades in inactive market for in portfolio securities	92.60%
Auction Rate Securities Owned [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	900
Valuation Adjustment	92
Fair Value	808
Valuation Technique	Discounted Cash Flow
Discount Rate	3.33%
Duration	7 years
Current Yield	1.67%
Auction Rate Securities Owned [Member] | Other Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	3,625
Valuation Adjustment	1,282
Fair Value	$ 2,343
Valuation Technique	Secondary Market Trading Activity
Observable trades in inactive market for in portfolio securities	64.60%
Auction Rate Securities Owned [Member] | Minimum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.37%
Duration	4 years
Current Yield	0.13%
Auction Rate Securities Owned [Member] | Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	1.86%
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	1.62%
Duration	4 years
Current Yield	0.28%
Auction Rate Securities Owned [Member] | Weighted Average [Member] | Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.51%
Duration	4 years 6 months
Current Yield	0.24%
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	3.33%
Duration	7 years
Current Yield	1.67%
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 Purchase Commitment [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
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	3.33%
Duration	7 years
Current Yield	1.67%
Auction Rate Securities Purchase Commitment [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.51%
Duration	4 years 6 months
Current Yield	0.24%
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	1.37%
Current Yield	0.17%
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	1.86%
Current Yield	0.43%
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	1.62%
Duration	4 years
Current Yield	0.28%
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	3.33%
Duration	7 years
Current Yield	1.67%
Auction Rate Securities Purchase Commitment [Member] | Weighted Average [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.51%
Duration	4 years 6 months
Current Yield	0.24%
Auction Rate Securities [Member] | Fair Value, Valuation Scenario Two [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Sensitivity Analysis Of Fair Value Of Interests Continued To Be Held By Transferor Servicing Assets Or Liabilities Change In Discount Rate	0.50%
Auction Rate Securities [Member] | Fair Value, Valuation Scenario One [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Sensitivity Analysis Of Fair Value Of Interests Continued To Be Held By Transferor Servicing Assets Or Liabilities Change In Discount Rate	0.25%