Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012
Income Tax Contingency [Line Items]
Liability for Uncertain Tax Positions, Current	$ 86,191	$ 102,171
Income Taxes Paid	143,111,000	132,874,000	$ 118,290,000
Unrecognized Tax Benefits	750,258	583,977	$ 113,000	$ 391,000
Deferred Tax Liabilities, Property, Plant and Equipment	6,425,000	8,585,000
deferred profit sharing contingent commissions	$ 10,567,000	$ 9,767,000
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	35.00%	35.00%	35.00%
Current Federal Tax Expense (Benefit)	$ 126,145,000	$ 118,490,000	$ 109,893,000
Current State and Local Tax Expense (Benefit)	21,110,000	17,625,000	15,482,000
Current Foreign Tax Expense (Benefit)	590,000	430,000	109,000
Current Income Tax Expense (Benefit)	147,845,000	136,545,000	125,484,000
Deferred Federal Income Tax Expense (Benefit)	15,551,000	18,416,000	5,987,000
Deferred State and Local Income Tax Expense (Benefit)	2,612,000	4,280,000	1,440,000
Deferred Foreign Income Tax Expense (Benefit)	$ 0	0	(58,000)
Deferred Foreign Income Tax Expense (Benefit)	2566000
Summary of Income Tax Contingencies [Table Text Block]	$ 18,163,000	22,696,000	7,369,000
Income Tax Expense (Benefit)	$ 166,008,000	$ 159,241,000	$ 132,853,000
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	3.90%	3.90%	3.30%
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%	0.40%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Percent	(0.30%)	0.10%	0.10%
Effective Income Tax Rate Reconciliation, Percent	39.20%	39.60%	39.10%
Deferred Tax Assets Operating Loss Carryforwards Current	$ 10,000	$ 10,000
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals	14,032,000	14,858,000
Deferred Tax Assets, Net, Current	24,609,000	24,635,000
Net Unrealized Holding Gain on Available for Sale Securities	(12,000)	(9,000)
Deferred Tax Liabilities, Goodwill and Intangible Assets	422,478,000	393,251,000
Non Current Deferred Tax Liability	428,891,000	401,827,000
Deferred income taxes, net	382,295,000	360,949,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation	44,912,000	38,966,000
Deferred Tax Assets, Operating Loss Carryforwards	2,384,000	2,518,000
Deferred Tax Assets, Valuation Allowance	(700,000)	(606,000)
Deferred Tax Assets, Net, Noncurrent	46,596,000	40,878,000
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions	412,000	773,000	$ 0
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	(41,000)	0	(21,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	(205,000)	$ (302,000)	$ (257,000)
Operating Loss Carryforwards	156,435
Operating Loss Carryforwards State	$ 60,183,125
Operating Loss Carryforwards, Expiration Date	2036