Retirement Agreements and Other Postretirement Benefit Plan (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) h	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
DefinedContributionPlanMinimumNumberOfMonthsEmployedEligibility	3 months
Minimum number of years employed in order to participate in the 401(k) plan	1 year
Minimum number of hours worked annually in order to participate in the 401(k) plan | h	1,000
Defined contribution plan contribution percent	3.00%
Expenses related to the 401(k) plan	$ 1,577,000	$ 810,000	$ 741,000
Amount accrued for all deferred compensation plan agreements	9,492,000	8,487,000
Amount recognized in accumulated other comprehensive income, before tax	(87,191)	(165,437)	(173,812)
Amount recognized in accumulated other comprehensive income, net of tax	(57,556)	$ (109,200)	$ (114,726)
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2018	11,789
2019	13,343
2020	14,913
2021	16,501
2022	24,250
Next 5 years thereafter	$ 174,283
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Noncompete period following employment	2 years