Basis of Financial Reporting Adopted Guidance (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Premiums Commissions And Fees Receivable	$ 736,341		$ 736,341		$ 699,460	$ 546,402
Other Assets, Current	106,724		106,724		100,544	47,864
Insurance Commissions and Fees	472,068	$ 464,724	972,406	$ 909,290
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			(224)
Retained Earnings (Accumulated Deficit)	2,697,396		2,697,396		2,574,114	2,456,599
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification						12,107
Premiums Payable To Insurance Companies	815,504		815,504		697,270	685,163
Accounts Payable, Current	98,265		98,265		72,924	64,177
Accrued Liabilities, Current	217,898		217,898		251,542	228,748
Deferred Tax Liabilities, Net, Noncurrent	284,309		284,309		300,760	256,185
Employee Compensation And Benefits	251,958	244,517	522,857	490,383
Other Cost and Expense, Operating	83,694	71,312	160,007	138,231
Income Tax Expense (Benefit)	26,988	41,900	54,601	82,757
Net Income (Loss) Attributable to Parent	73,922	$ 66,102	164,750	$ 136,212
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income			(1,758)
Accounts Payable [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification						8,747
Other Liabilities [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification						22,794
Deferred Income Tax Charge [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification						44,575
Employee Compensation and Benefits [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			2,119
Operating Expense [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			(4,684)
Income Tax Expense [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			$ (583)
Retained Earnings [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification					$ 117,500	117,515
Other Current Assets [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification						52,680
Premiums Receivable [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification						$ 153,058
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Insurance Commissions and Fees	972,182
Employee Compensation And Benefits	524,976
Other Cost and Expense, Operating	155,323
Income Tax Expense (Benefit)	55,184
Net Income (Loss) Attributable to Parent	$ 166,508