Retirement Plans - Reconciliation of Level 3 Partnership Assets Within Fair Value Hierarchy (Details 12) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Level 3 fund assets
Beginning Balance	$ 293,567
Ending Balance	286,539	$ 293,567
Partnership
Reconciliation of Level 3 fund assets
Beginning Balance	12,862
Ending Balance	10,787	12,862
Significant Unobservable Inputs (Level 3) [Member]
Reconciliation of Level 3 fund assets
Beginning Balance	12,862
Ending Balance	10,787	12,862
Significant Unobservable Inputs (Level 3) [Member] | Partnership
Reconciliation of Level 3 fund assets
Beginning Balance	12,862	13,360
Capital contributions	343	1,419
Realized and unrealized gain	2,107	584
Capital distributions	(4,525)	(2,501)
Ending Balance	$ 10,787	$ 12,862