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Retirement Benefit Plans (Narrative) (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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 |  | 101,165,000 | 40,138,000 |
Funded status at end of year-net asset (liability) |  | 170,272,000 | 50,601,000 |
Excess of plan assets over the benefit obligation |  | 185,800,000 |  |
Unfunded benefit obligation |  | 15,500,000 |  |
Estimated amounts of actuarial loss that will be amortized from AOCI into net periodic benefit cost over the next year |  | $ 5,100,000 |  |
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 |  | $ 15,700,000 | $ 14,600,000 | $ 8,400,000
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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