Retirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Hancock Plans [Member]
Weighted average assumptions as of measurement date
Hancock plan	3.83%	4.25%
Whitney Plan [Member]
Weighted average assumptions as of measurement date
Hancock plan	3.55%	4.10%
Defined Benefit Plan Postretirement [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost	$ 192	$ 137
Interest cost	1,337	1,091
Other changes in plan assets and benefit obligations recognized in other comprehensive income, before taxes
Amount of loss recognized during the year	(918)	(489)
Net actuarial loss/(gain)mm	4,435	5,939
Net periodic postretirement benefit cost
Service cost	192	137
Interest costs	1,337	1,091
Defined Benefit Plan Postretirement [Member] | Hancock Plans [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net periodic benefit cost	2,447	1,718	934
Service cost	192	137	124
Interest cost	1,337	1,091	556
Net periodic benefit cost	2,447	1,718	934
Other changes in plan assets and benefit obligations recognized in other comprehensive income, before taxes
Amount of loss recognized during the year	(966)	(538)	(302)
Net actuarial (gain)/loss	4,435	5,938	2,098
Amortization of prior service cost	48	48	48
Total recognized in other comprehensive income	3,517	5,448	1,844
Total recognized in net periodic benefit cost and other comprehensive income	5,964	7,166	2,778
Total recognized in net periodic benefit cost and other comprehensive income	5,964	7,166	2,778
Net periodic postretirement benefit cost
Service cost	192	137	124
Interest costs	1,337	1,091	556
Amortization of net loss	966	538	302
Amortization of prior service cost	(48)	(48)	(48)
Net periodic benefit cost	2,447	1,718	934
Qualified [Member] | Defined Benefit Plan-Pension [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net periodic benefit cost	10,586	7,782	6,368
Service cost	12,940	8,440	3,500
Interest cost	16,518	12,117	5,233
Expected return on plan assets	(25,398)	(15,118)	(4,646)
Recognized net amortization and deferral	6,526	2,343	2,281
Net periodic benefit cost	10,586	7,782	6,368
Other changes in plan assets and benefit obligations recognized in other comprehensive income, before taxes
Amount of loss recognized during the year	(6,526)	(2,343)	(2,281)
Net actuarial loss/(gain)mm	(8,402)	87,313	4,338
Total recognized in other comprehensive income	(14,928)	84,970	2,057
Total recognized in net periodic benefit cost and other comprehensive income	(4,342)	92,752	8,425
Total recognized in net periodic benefit cost and other comprehensive income	(4,342)	92,752	8,425
Net periodic postretirement benefit cost
Service cost	12,940	8,440	3,500
Interest costs	16,518	12,117	5,233
Net periodic benefit cost	10,586	7,782	6,368
Qualified [Member] | Defined Benefit Plan-Pension [Member] | Hancock Plans [Member]
Weighted average assumptions as of measurement date
Hancock plan	3.74%	4.35%	5.46%
Hancock plan	4.35%	5.46%	5.95%
Hancock plan	7.50%	7.50%	7.50%
Hancock plan	4.00%	4.00%	4.00%
Qualified [Member] | Defined Benefit Plan-Pension [Member] | Whitney Plan [Member]
Weighted average assumptions as of measurement date
Hancock plan	3.87%	4.31%
Hancock plan	4.31%	5.35%
Hancock plan	7.50%	7.50%
Hancock plan	3.58%	3.58%
Non Qualified [Member] | Defined Benefit Plan-Pension [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net periodic benefit cost	793	465
Service cost	49	27
Interest cost	688	438
Recognized net amortization and deferral	56
Net periodic benefit cost	793	465
Other changes in plan assets and benefit obligations recognized in other comprehensive income, before taxes
Amount of loss recognized during the year	(56)
Net actuarial loss/(gain)mm	1,401	1,597
Total recognized in other comprehensive income	1,345	1,597
Total recognized in net periodic benefit cost and other comprehensive income	2,138	2,062
Total recognized in net periodic benefit cost and other comprehensive income	2,138	2,062
Net periodic postretirement benefit cost
Service cost	49	27
Interest costs	688	438
Net periodic benefit cost	$ 793	$ 465
Non Qualified [Member] | Defined Benefit Plan-Pension [Member] | Whitney Plan [Member]
Weighted average assumptions as of measurement date
Hancock plan	3.87%	4.31%
Hancock plan	4.31%	5.35%
Hancock plan	3.58%	3.58%