Basis of Financial Reporting Adopted Guidance (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents, at Carrying Value	$ 432,548		$ 477,928		$ 432,548	$ 477,928		$ 438,961	$ 573,383
Assets	7,164,767	$ 7,164,767	5,701,557		7,164,767	5,701,557	$ 6,863,968	6,688,668
Premiums Commissions And Fees Receivable	913,910				913,910			844,815
Other Assets, Current	131,386				131,386		125,712	128,716
Insurance Commissions and Fees	572,932		472,068		1,190,395	972,406
Retained Earnings (Accumulated Deficit)	2,995,206				2,995,206			2,833,622
Premiums Payable To Insurance Companies	979,417				979,417			857,559
Accounts Payable, Current	98,869				98,869			87,345
Accrued Liabilities, Current	259,068				259,068		293,146	279,310
Deferred Tax Liabilities, Net, Noncurrent	310,510				310,510			315,732
Employee Compensation And Benefits	309,610		251,958		642,447	522,857
Other Cost and Expense, Operating	98,050		83,694		186,833	160,007
Income Tax Expense (Benefit)	30,929		26,988		65,553	54,601
Net Income (Loss) Attributable to Parent	92,593	113,896	73,922	$ 90,828	206,489	164,750
Operating Lease, Liability, Noncurrent	169,555				169,555		161,464	0
Liabilities							3,863,400	3,688,100
Investment Income, Nonoperating	1,525		731		2,606	1,332
Other Income	762		388		1,498	910
Revenues	575,219		473,187		1,194,499	974,648
Gain (Loss) on Disposition of Assets	1,016		230		511	2,650
Amortization	25,954		20,785		52,146	41,324
Depreciation	5,666		5,599		11,701	11,151
Interest Expense	16,293		10,052		31,491	19,723
Change In Estimated Acquisition Earn Out Payable	(2,860)		419		(1,650)	2,885
Operating Expenses	451,697		372,277		922,457	755,297
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	$ 123,522		$ 100,910		$ 272,042	$ 219,351
Earnings Per Share, Basic	$ 0.33		$ 0.27		$ 0.73	$ 0.60
Earnings Per Share, Diluted	0.33		0.26		0.73	0.58
Common Stock, Dividends, Per Share, Declared	$ 0.080		$ 0.075		$ 0.160	$ 0.150
Restricted Cash and Investments, Current	$ 382,233				$ 382,233			338,635
Short-term Investments	10,542				10,542			12,868
Reinsurance Recoverables, Gross	77,335				77,335			65,396
Prepaid Reinsurance Premiums	343,594				343,594			337,920
Assets, Current	2,291,548				2,291,548			2,167,311
Property, Plant and Equipment, Net	122,511				122,511			100,395
Goodwill	3,551,661				3,551,661			3,432,786
Intangible Assets, Net (Excluding Goodwill)	899,583				899,583			898,807
Investments	24,896				24,896			17,394
Other Assets, Noncurrent	90,541				90,541			71,975
Liability for Claims and Claims Adjustment Expense	77,335				77,335			65,212
Unearned Premiums	343,594				343,594			337,920
Premium Deposits And Credits Due Customers	93,827				93,827			105,640
Long-term Debt, Current Maturities	55,000				55,000			50,000
Liabilities, Current	1,907,110				1,907,110			1,782,986
Long-term Debt, Excluding Current Maturities	1,426,777				1,426,777			1,456,990
Other Liabilities, Noncurrent	170,184				170,184			132,392
Common Stock, Value, Issued	29,582				29,582			29,338
Additional Paid in Capital, Common Stock	653,415				653,415			615,180
Treasury Stock, Value	497,572				497,572			477,572
Stockholders' Equity Attributable to Parent	3,180,631	$ 3,098,431	$ 2,831,482	$ 2,770,593	3,180,631	$ 2,831,482		3,000,568	$ 2,582,699
Liabilities and Equity	$ 7,164,767				$ 7,164,767			$ 6,688,668
Accounting Standards Update 2016-02 [Member]
Assets							175,300
Other Assets, Current							(3,004)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification							202,900
Accrued Liabilities, Current							13,836
Operating Lease, Liability, Noncurrent							161,464
Liabilities							$ 175,300