Fair Value Measurements - Changes in Level 3 Securities (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 96,886	$ 36,597
Purchases	166,339	86,378
Maturities and calls	(116,054)	(40,285)
Amortization	(35)	(4)
Changes in fair value	0	0
Ending Balance	147,136	82,686
Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	3,820	3,820
Changes in fair value	0	0
Ending Balance	3,820	3,820
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	92,964	32,487
Purchases	166,339	86,378
Maturities and calls	(115,989)	(40,206)
Amortization	(35)	(4)
Changes in fair value	0	0
Ending Balance	143,279	78,655
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	102	290
Maturities and calls	(65)	(79)
Changes in fair value	0	0
Ending Balance	$ 37	$ 211