Retirement Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Whitney Plans [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 152	$ 205	$ 456	$ 274
Net periodic benefit cost	152	205	456	274
Whitney Plans [Member] | Pension Benefits [Member] | Qualified Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,533	1,607	4,597	2,144
Interest cost	2,645	2,856	7,937	3,808
Expected return on plan assets	(4,249)	(4,126)	(12,747)	(5,502)
Amortization of net loss	411		1,234
Net periodic benefit cost	340	337	1,021	450
Whitney Plans [Member] | Pension Benefits [Member] | Nonqualified Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	13	12	37	16
Interest cost	172	188	516	250
Amortization of net loss	14		42
Net periodic benefit cost	199	200	595	266
Hancock Plans [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	48	34	144	103
Interest cost	209	153	627	458
Amortization of prior service cost	(13)	(13)	(41)	(40)
Amortization of net loss	176	135	530	404
Amortization of transition obligation	1	1	4	4
Net periodic benefit cost	421	310	1,264	929
Hancock Plans [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,703	1,173	5,109	3,517
Interest cost	1,484	1,363	4,451	4,089
Expected return on plan assets	(2,101)	(1,372)	(6,302)	(4,117)
Amortization of net loss	1,220	585	3,660	1,757
Net periodic benefit cost	$ 2,306	$ 1,749	$ 6,918	$ 5,246