Fair Value of Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended						9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 State and local governments [Member]	Dec. 31, 2013 State and local governments [Member]	Sep. 30, 2014 Level 3 [Member]	Dec. 31, 2013 Level 3 [Member]	Sep. 30, 2014 Level 3 [Member] State and local governments [Member]	Sep. 30, 2013 Level 3 [Member] State and local governments [Member]	Sep. 30, 2014 Level 3 [Member] State and local governments [Member] Tax - Exempt [Member]	Sep. 30, 2013 Level 3 [Member] State and local governments [Member] Tax - Exempt [Member]	Sep. 30, 2014 Level 3 [Member] State and local governments [Member] Taxable [Member]	Sep. 30, 2013 Level 3 [Member] State and local governments [Member] Taxable [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 324,509		$ 77,748	$ 81,473	$ 7,907	$ 10,346	$ 10,346	$ 16,408	$ 8,802	$ 14,863	$ 1,544	$ 1,545
Change in Market Value							(1,619)	(497)	(1,262)	(497)	(357)	0
Purchases	0	83,310						519		519		0
Payments & Maturities							(820)	(4,388)	(820)	(4,388)	0	0
Ending Balance	$ 253,119		$ 77,748	$ 81,473	$ 7,907	$ 10,346	$ 7,907	$ 12,042	$ 6,720	$ 10,497	$ 1,187	$ 1,545