Qualified Employee Benefit Plans - Reconciliation of Amounts Recognized in Other Comprehensive Income for the Retirement Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial (loss)	$ (3,190)	$ (3,043)	$ 3,267
Amortization of prior service cost	24	93	(895)
Changes in employee benefit related items	(3,166)	(2,950)	2,372
Income tax expense	(27)	(22)	(165)
Employee benefit related items, net of tax	(3,193)
Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial (loss)	(3,043)
Amortization of prior service cost	24	93	(895)
Changes in employee benefit related items	(3,019)	(3,022)	1,987
Income tax expense	(49)
Employee benefit related items, net of tax	(3,068)	$ (3,032)	$ 1,888
Retired Individual Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial (loss)	(20)
Amortization of prior service cost	0
Changes in employee benefit related items	(20)
Income tax expense	(1)
Employee benefit related items, net of tax	(21)
Foreign Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial (loss)	(127)
Amortization of prior service cost	0
Changes in employee benefit related items	(127)
Income tax expense	23
Employee benefit related items, net of tax	$ (104)