Retirement Plans Retirement Plans - Net Periodic Pension Cost - SERP (Details) - SERP - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0	$ 0
Interest cost	287	323	278
Net amortization and deferral	(211)	(343)	(337)
Total net periodic pension cost	$ 498	$ 666	$ 615