Components of Net Periodic Benefit Cost (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018
Net pension cost of plans
Interest cost	$ 1,685,000	$ 1,837,000	$ 3,370,000	$ 3,674,000
Expected return on plan assets	(1,524,000)	(1,832,000)	(3,047,000)	(3,664,000)
Recognized actuarial loss	689,000	688,000	1,377,000	1,377,000
Net periodic benefit cost	850,000	$ 693,000	1,700,000	$ 1,387,000
Restoration Pension Plan
Defined Benefit Plan Disclosure
Benefits paid	$ 400,000		$ 800,000
Forecast | Qualified Pension Plan
Defined Benefit Plan Disclosure
Contributions to qualified pension plan					$ 0