New Accounting Pronouncements (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Insurance Commissions and Fees									$ 2,009,857,000	$ 1,857,270,000	$ 1,762,787,000
Premiums Commissions And Fees Receivable	$ 844,815,000				$ 546,402,000				844,815,000	546,402,000		$ 699,460,000
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									567,529,000	441,975,000	411,042,000
Insurance Commissions									1,362,037,000
Fees and Commissions, Other									(530,838,000)
Other Assets, Current	128,716,000				47,864,000				128,716,000	47,864,000		100,544,000
Premiums Payable To Insurance Companies	857,559,000				685,163,000				857,559,000	685,163,000		697,270,000
Accounts Payable, Current	87,345,000				64,177,000				87,345,000	64,177,000		72,924,000
Accrued Liabilities, Current	279,310,000				228,748,000				279,310,000	228,748,000		251,542,000
Deferred Tax Liabilities, Net, Noncurrent	315,732,000				256,185,000				315,732,000	256,185,000		300,760,000
Retained Earnings (Accumulated Deficit)	2,833,622,000				2,456,599,000				2,833,622,000	2,456,599,000		2,574,114,000
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification									(18,399,000)
Employee Compensation And Benefits									1,068,914,000	994,652,000	925,217,000
Other Cost and Expense, Operating									332,118,000	283,470,000	262,872,000
Income Tax Expense (Benefit)									118,207,000	50,092,000	166,008,000
Net Income (Loss) Attributable to Parent	73,452,000	$ 106,053,000	$ 73,922,000	$ 90,828,000	187,505,000	$ 75,913,000	$ 66,102,000	$ 70,110,000	344,255,000	399,630,000	257,491,000
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income									12,367,000
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Insurance Commissions and Fees									1,991,458,000
Employee Compensation And Benefits									1,077,749,000
Other Cost and Expense, Operating									321,497,000
Income Tax Expense (Benefit)									113,961,000
Net Income (Loss) Attributable to Parent									331,888,000
Accounting Standards Update 2016-18 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by (Used in) Operating Activities, Continuing Operations											375,200,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Insurance Commissions									9,900,000
Profit-Sharing Contingent Commission Revenue									(2,300,000)
Fees and Commissions, Other									6,200,000
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption												117,500,000
Net Income (Loss) Attributable to Parent									344,255,000	399,630,000	$ 257,491,000
Retained Earnings [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					117,515,000					117,515,000
Services [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fees and Commissions, Other									10,500,000
Various [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fees and Commissions, Other									(4,300,000)
Accounts Payable [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					8,747,000					8,747,000
Accrued Liabilities [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					22,794,000					22,794,000
Deferred Income Tax Charge [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					44,575,000					44,575,000
Cost to Obtain [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Costs	$ 13,700,000								13,700,000
Capitalized Contract Cost, Amortization									500,000
Employee Compensation and Benefits [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification									8,835,000
Operating Expense [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification									(10,621,000)
Income Tax Expense [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification									4,246,000
Cost to Fulfill [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Costs												$ 52,700,000
Capitalized Contract Cost, Amortization									$ 1,300,000
Premiums Receivable [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					153,058,000					153,058,000
Other Current Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					52,700,000					52,700,000
Other Current Assets [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					$ 52,680,000					$ 52,680,000