Retirement Plans - Reconciliation of Level 3 Hedge Fund Asset Within Fair Value Hierarchy (Details 11) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Reconciliation of Level 3 fund assets
Beginning Balance	$ 293,567
Ending Balance	286,539
International hedge funds
Reconciliation of Level 3 fund assets
Beginning Balance	23,542
Ending Balance	6,604
Significant Unobservable Inputs (Level 3)
Reconciliation of Level 3 fund assets
Beginning Balance	12,862
Ending Balance	10,787
Significant Unobservable Inputs (Level 3) | International hedge funds
Reconciliation of Level 3 fund assets
Beginning Balance	0
Ending Balance	$ 0