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Retirement Benefit Plans (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Contributed to pension plan	$ 100,000,000
Newly eligible associates initial savings rate		3.00%
First 1% Of Contribution Saved [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching percentage		100.00%
Percentage of compensation saved		1.00%
Next 5% Of Contribution Saved [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching percentage		50.00%
Percentage of compensation saved		5.00%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributed to pension plan		$ 0	$ 0
Employer contributions		1,178,000	101,165,000
Funded status at end of year-net asset (liability)		154,995,000	170,272,000
Excess of plan assets over the benefit obligation		171,200,000
Unfunded benefit obligation		$ 16,200,000
2017 Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributed to pension plan				$ 39,000,000
Amended Hancock 401K Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional matching percentage		2.00%
Period of employment for eligibility		3 years
Amended Hancock 401K Plan [Member] | 2% Of Contribution [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching percentage		2.00%
Amended Hancock 401K Plan [Member] | 4% Of Contribution [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching percentage		4.00%
Amended Hancock 401K Plan [Member] | 6% Of Contribution [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching percentage		6.00%
Whitney 401K Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 17,400,000	$ 15,700,000	$ 14,600,000
Minimum age for increase in per capita cost of health care benefit		55 years
Years of credited service reaching 55 years of age		10 years
Hancock Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum age for increase in per capita cost of health care benefit		55 years
Years of credited service reaching 55 years of age		10 years
Increase (decrease) in pre- and post-Medicare age health costs rate		7.25%	7.50%
Period of assumed health rate decline		4 years	4 years
Decrease in ultimate rate over a period of time		6.25%	6.75%

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