Note 11 - Retirement Plans (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Stock, Capital Shares Reserved for Future Issuance	6,130,944	7,078,916
Pension Plan [Member] | Gray Pension Plan [Member]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 135	$ 117
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year	3
Capital Accumulation Plan [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	11	6	$ 7
Defined Contribution Plan, Expected Contributions In Next Twelve Months	$ 12
Common Stock, Capital Shares Reserved for Future Issuance	1,391,210
Capital Accumulation Plan [Member] | First 3% of Each Employee's Salary Deferral [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	1.00%
Capital Accumulation Plan [Member] | Next 2% of Each Employee's Salary Deferral [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	5.00%
Profit Sharing Contribution [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 5	$ 4	$ 4