Qualified Employee Benefit Plans - Amounts Recognized in Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Prior service cost	$ 24	$ 93	$ (895)
Changes in employee benefit related items	(3,166)	(2,950)	2,372
Employee benefit related items, net of tax	(3,193)
Retirement Plan [Member]
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Unrecognized net (loss) gain from experience different from that assumed and effects of changes and assumptions	(3,043)	(3,115)	2,882
Prior service cost	24	93	(895)
Changes in employee benefit related items	(3,019)	(3,022)	1,987
Income tax expense	(49)	(10)	(99)
Employee benefit related items, net of tax	$ (3,068)	$ (3,032)	$ 1,888