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, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial gain (loss)	$ 1,586	$ (3,190)	$ (3,043)
Amortization of prior service cost	24	24	93
Changes in employee benefit related items	1,610	(3,166)	(2,950)
Income tax expense	(139)	(27)	(22)
Other comprehensive income (loss)	1,471
Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial gain (loss)	1,870
Amortization of prior service cost	24	24	93
Changes in employee benefit related items	1,894	(3,019)	(3,022)
Income tax expense	(207)
Other comprehensive income (loss)	1,687	$ (3,068)	$ (3,032)
Retired Individual Plan
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial gain (loss)	53
Amortization of prior service cost	0
Changes in employee benefit related items	53
Income tax expense	(2)
Other comprehensive income (loss)	51
Foreign Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Recognized actuarial gain (loss)	(337)
Amortization of prior service cost	0
Changes in employee benefit related items	(337)
Income tax expense	70
Other comprehensive income (loss)	$ (267)