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Supplemental Executive Retirement Plans - Defined Contribution Supplemental Executive Retirement Plan (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Supplemental Executive Retirement Plans    
Estimated accumulated benefit obligation $ 3.5 $ 3.0
Funded estimated accumulated benefit obligation 3.0 2.4
Unfunded estimated accumulated benefit obligation $ 0.5 $ 0.6