Fair Value Measurements - Changes in Level 3 Securities (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 53,782	$ 18,914
Purchases	43,807	79,588
Maturities and calls	(57,087)	(34,408)
Amortization	(23)	(33)
Ending Balance	40,479	64,061
Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	3,963	3,725
Purchases
Maturities and calls
Amortization
Ending Balance	3,963	3,725
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	49,477	14,772
Purchases	43,807	79,588
Maturities and calls	(57,036)	(34,333)
Amortization	(23)	(33)
Ending Balance	36,225	59,994
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	342	417
Maturities and calls	(51)	(75)
Ending Balance	$ 291	$ 342