Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 107,872
Valuation Adjustment	8,514
Fair Value	99,358
Auction Rate Securities Purchase Commitment [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	13,072
Valuation Adjustment	1,037
Fair Value	12,035
Auction Rate Securities Purchase Commitment [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	7,582
Valuation Adjustment	424
Fair Value	7,158
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	538
Valuation Adjustment	80
Fair Value	458
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	4,952
Valuation Adjustment	533
Fair Value	4,419
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Owned [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	94,800
Valuation Adjustment	7,477
Fair Value	87,323
Auction Rate Securities Owned [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	74,400
Valuation Adjustment	4,392
Fair Value	70,008
Valuation Technique	Discounted Cash Flow
Auction Rate Securities Owned [Member] | Municipal Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	10,400
Valuation Adjustment	1,118
Fair Value	9,282
Valuation Technique	Discounted Cash Flow
Discount Rate	2.76%
Duration	4 years 6 months
Current Yield	0.19%
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	624
Fair Value	5,351
Valuation Technique	Secondary Market Trading Activity
Observable trades in inactive market for in portfolio securities	89.60%
Auction Rate Securities Owned [Member] | Student Loan Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	400
Valuation Adjustment	60
Fair Value	340
Valuation Technique	Discounted Cash Flow
Discount Rate	3.40%
Duration	7 years
Current Yield	0.98%
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	1,283
Fair Value	$ 2,342
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	1.54%
Duration	4 years
Current Yield	0.09%
Auction Rate Securities Owned [Member] | Maximum [Member] | Auction Rate Preferred Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.10%
Current Yield	0.46%
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	1.82%
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	2.76%
Duration	4 years 6 months
Current Yield	0.19%
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	89.60%
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	3.40%
Duration	7 years
Current Yield	0.98%
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	3.40%
Duration	7 years
Current Yield	0.98%
Auction Rate Securities Purchase Commitment [Member] | Municipal Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.76%
Duration	4 years 6 months
Current Yield	0.19%
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	1.54%
Current Yield	0.09%
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	2.10%
Current Yield	0.46%
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	1.82%
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	3.40%
Duration	7 years
Current Yield	0.98%
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	2.76%
Duration	4 years 6 months
Current Yield	0.19%
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%