Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Income Tax Contingency [Line Items]
Operating Loss Carryforwards State	$ 52,200,000
Operating Loss Carryforwards	100,000
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	(126,000)	$ (205,000)	$ (302,000)
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	0	(41,000)	0
Deferred Tax Assets, Valuation Allowance	(893,000)	(700,000)
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions	1,070,000	412,000	773,000
Deferred income taxes, net	256,185,000	357,686,000
Deferred Tax Assets, Operating Loss Carryforwards	2,434,000	2,394,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation	36,701,000	44,912,000
Liability for Uncertain Tax Positions, Current	228,608	86,191	102,171
Income Taxes Paid	$ 152,024,000	143,111,000	132,874,000
Document Period End Date	Dec. 31, 2017
Unrecognized Tax Benefits	$ 1,694,000	750,258	$ 583,977	$ 113,000
Deferred Tax Liabilities, Property, Plant and Equipment	2,723,000	6,425,000
deferred profit sharing contingent commissions	$ 7,107,000	$ 10,567,000
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	35.00%	35.00%	35.00%
Current Federal Tax Expense (Benefit)	$ 129,954,000	$ 126,145,000	$ 118,490,000
Current State and Local Tax Expense (Benefit)	21,392,000	21,110,000	17,625,000
Current Foreign Tax Expense (Benefit)	929,000	590,000	430,000
Deferred Federal Income Tax Expense (Benefit)	18,999,000	15,551,000	18,416,000
Current Income Tax Expense (Benefit)	152,275,000	147,845,000	136,545,000
Deferred State and Local Income Tax Expense (Benefit)	2,984,000	2,612,000	4,280,000
Deferred Foreign Income Tax Expense (Benefit)	$ 0	0	0
Deferred Foreign Income Tax Expense (Benefit)	As of December 31, 2017, the Company has estimated $20.9 million of cash and cash equivalent balances related to accumulated earnings associated with our international operations.
Summary of Income Tax Contingencies [Table Text Block]	$ (102,183,000)	18,163,000	22,696,000
Income Tax Expense (Benefit)	$ (50,092,000)	$ (166,008,000)	$ (159,241,000)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	3.80%	3.90%	3.90%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent	0.30%	0.30%	0.30%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Meals and Entertainment, Percent	0.30%	0.30%	0.30%
Tax Reform Act Deferred Tax Revaluation and Transition Tax Impact	(26.90%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Percent	(1.40%)	(0.30%)	0.10%
Effective Income Tax Rate Reconciliation, Percent	11.10%	39.20%	39.60%
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals	$ 7,534,000	$ 14,032,000
Net Unrealized Holding Gain on Available for Sale Securities	(6,000)	(12,000)
Deferred Tax Liabilities, Goodwill and Intangible Assets	306,351,000	422,478,000
Non Current Deferred Tax Liability	309,068,000	428,891,000
Deferred Tax Assets, Net, Noncurrent	$ 52,883,000	$ 71,205,000