Retirement Plans - Benefit Obligations and Plan Expenses (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Average estimated duration of benefit cash flows	6 years 2 months 4 days
Contributions to plan during the year	$ 0	$ 0	$ 0
Pension Plans | Maximum
Defined Benefit Plan Disclosure [Line Items]
Eligible contributions by employer	10,000
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Contributions to plan during the year	377	$ 392	$ 332
Expected contribution	$ 384