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Fair Value of Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance			$ 248,492
Ending Balance	246,070		248,492
State and Local Governments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance			74,303
Ending Balance	77,708		74,303
Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance			7,931
Ending Balance	7,837		7,931
Level 3 [Member] \| State and Local Governments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	7,931	10,346
Change in Market Value	106	(1,490)
Payments & Maturities	(200)	(200)
Ending Balance	7,837	8,656
Level 3 [Member] \| State and Local Governments [Member] \| Tax - Exempt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	6,638	8,802
Change in Market Value	(3)	(1,121)
Payments & Maturities	(200)	(200)
Ending Balance	6,435	7,481
Level 3 [Member] \| State and Local Governments [Member] \| Taxable [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,293	1,544
Change in Market Value	109	(369)
Ending Balance	$ 1,402	$ 1,175