Basis of Financial Reporting Text Block (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 322,477	$ 558,248		$ 438,961	$ 573,383
Insurance Commissions and Fees	617,463	500,338
Other Assets, Current	146,492		$ 125,712	128,716
Operating Lease, Right-of-Use Asset	174,580		178,304	0
Assets	6,806,289	5,615,083	6,863,968	6,688,668
Accrued Liabilities, Current	193,986		293,146	279,310
Operating Lease, Liability, Noncurrent	159,823		161,464	0
Liabilities			3,863,400	3,688,100
Investment Income, Nonoperating	1,081	601
Other Income	736	522
Revenues	619,280	501,461
Employee Compensation And Benefits	332,837	270,899
Other Cost and Expense, Operating	88,783	76,313
Gain (Loss) on Disposition of Assets	(505)	2,420
Amortization	26,192	20,539
Depreciation	6,035	5,552
Interest Expense	15,198	9,671
Change In Estimated Acquisition Earn Out Payable	1,210	2,466
Operating Expenses	470,760	383,020
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	148,520	118,441
Income Tax Expense (Benefit)	34,624	27,613
Net Income (Loss) Attributable to Parent	$ 113,896	$ 90,828
Earnings Per Share, Basic	$ 0.41	$ 0.33
Earnings Per Share, Diluted	0.40	0.32
Common Stock, Dividends, Per Share, Declared	$ 0.080	$ 0.075
Restricted Cash and Investments, Current	$ 302,410			338,635
Short-term Investments	10,518			12,868
Premiums Commissions And Fees Receivable	847,132			844,815
Reinsurance Recoverables, Gross	51,356			65,396
Prepaid Reinsurance Premiums	314,363			337,920
Assets, Current	1,994,748			2,167,311
Property, Plant and Equipment, Net	117,364			100,395
Goodwill	3,513,218			3,432,786
Intangible Assets, Net (Excluding Goodwill)	907,638			898,807
Investments	17,528			17,394
Other Assets, Noncurrent	81,213			71,975
Premiums Payable To Insurance Companies	801,475			857,559
Liability for Claims and Claims Adjustment Expense	51,356			65,212
Unearned Premiums	314,363			337,920
Premium Deposits And Credits Due Customers	94,426			105,640
Accounts Payable, Current	136,467			87,345
Long-term Debt, Current Maturities	55,000			50,000
Liabilities, Current	1,647,073			1,782,986
Long-term Debt, Excluding Current Maturities	1,439,998			1,456,990
Deferred Tax Liabilities, Net, Noncurrent	300,857			315,732
Other Liabilities, Noncurrent	160,107			132,392
Common Stock, Value, Issued	29,584			29,338
Additional Paid in Capital, Common Stock	621,249			615,180
Treasury Stock, Value	477,572			477,572
Retained Earnings (Accumulated Deficit)	2,925,170			2,833,622
Stockholders' Equity Attributable to Parent	3,098,431	$ 2,770,593		3,000,568	$ 2,582,699
Liabilities and Equity	6,806,289			$ 6,688,668
Share-based Compensation	$ 12,994	$ 7,295
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets, Current			(3,004)
Cumulative Effect of New Accounting Principle in Period of Adoption			202,900
Operating Lease, Right-of-Use Asset			178,304
Assets			175,300
Accrued Liabilities, Current			13,836
Operating Lease, Liability, Noncurrent			161,464
Liabilities			$ 175,300