Employee Benefit Plans - Qualified Pension Plan (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans
Estimated future benefit payments 2018		$ 17,800
Estimated future benefit payments 2019		2,200
Estimated future benefit payments 2020		2,200
Estimated future benefit payments 2021		2,100
Estimated future benefit payments 2022		2,100
Estimated future benefit payments, five years thereafter		9,000
Pension plans
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets		41,517	$ 42,061	$ 43,603
Expected future benefit and annuity contracts payments		43,300
Voluntary contribution	$ 800
Accumulated benefit obligation		$ 42,900	$ 42,000