Retirement Benefit Plans (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended			24 Months Ended
	Mar. 31, 2019	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Newly eligible associates initial savings rate			3.00%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributed to pension plan						$ 0
Employer contributions			$ 40,138,000	$ 1,132,000
Funded status at end of year-net asset (liability)			50,601,000	50,521,000		50,601,000
Excess of plan assets over the benefit obligation			65,100,000			65,100,000
Unfunded benefit obligation			14,500,000			14,500,000
Estimated amounts of actuarial loss that will be amortized from AOCI into net periodic benefit cost over the next year			$ 8,900,000			$ 8,900,000
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			7.50%			7.50%
Period of assumed health rate decline			4 years
Decrease in ultimate rate over a period of time			6.75%			6.75%
Whitney 401K Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions			$ 14,600,000	$ 8,400,000	$ 7,700,000
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
Amended Hancock Pension Plan And 401K Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Reduction of benefit obligation		$ 17,300,000
Amended Hancock 401K Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Period of employment for eligibility			3 years
Additional matching percentage		2.00%
2017 Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributed to pension plan			$ 39,000,000
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%
2% Of Contribution [Member] | Amended Hancock 401K Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching percentage		2.00%
4% Of Contribution [Member] | Amended Hancock 401K Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching percentage		4.00%
6% Of Contribution [Member] | Amended Hancock 401K Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching percentage		6.00%
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributed to pension plan	$ 100,000,000