CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Document Period End Date			Jun. 30, 2018
Payments of Debt Issuance Costs			$ 0	$ (2,753)
Cash, Cash Equivalents and Restricted Cash			787,159	876,930	$ 824,088	$ 781,283
Cash flows from operating activities:
Net income	$ 73,922	$ 66,102	164,750	136,212
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	20,785	21,347	41,324	42,967
Depreciation	5,599	5,655	11,151	11,753
Non-cash stock-based compensation			15,027	15,326
Change in estimated acquisition earn-out payables	419	5,589	2,885	9,617
Deferred income taxes			(16,437)	14,793
Amortization of debt discount			79	79
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Amortization Expense			740	943
Accretion (Amortization) of Discounts and Premiums, Investments			1	13
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables			(3,408)	(7,109)
Net gain on sales of investments, fixed assets and customer accounts			(2,432)	136
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable decrease			(29,471)	2,918
Reinsurance recoverables decrease			388,704	55,604
Prepaid reinsurance premiums decrease			10,310	10,925
Other assets decrease			(15,731)	(9,326)
Premiums payable to insurance companies (decrease)			106,839	59,219
Premium deposits and credits due customers increase			8,360	8,500
Losses and loss adjustment reserve (decrease)			(388,572)	(55,604)
Unearned premiums (decrease)			(10,310)	(10,925)
Accounts payable increase			31,343	38,510
Accrued expenses and other liabilities (decrease)			(34,837)	(26,021)
Other liabilities (decrease)			(2,909)	(28,449)
Net cash provided by operating activities			277,406	270,081
Cash flows from investing activities:
Additions to fixed assets	(9,639)	(5,816)	(19,390)	(8,848)
Payments for businesses acquired, net of cash acquired			(141,803)	(11,526)
Proceeds from sales of fixed assets and customer accounts			2,906	669
Purchases of investments			(8,863)	(5,916)
Proceeds from Sale, Maturity and Collection of Investments			16,346	2,911
Net cash used in investing activities			(150,804)	(22,710)
Cash flows from financing activities:
Payments on acquisition earn-outs			(5,183)	(8,668)
Payments on long-term debt			(110,001)	(86,750)
Issuances of common stock for employee stock benefit plans			720	500
Repurchase Of Stock Benefit Plan Shares For Employees To Fund Tax Withholdings			(7,656)	(5,054)
Purchase of treasury stock			(11,250)	(11,159)
Settlement (prepayment) of accelerated share repurchase program			11,250	0
Cash dividends paid			(41,411)	(37,840)
Net cash used in financing activities			(163,531)	(151,724)
Net increase in cash and cash equivalents inclusive of restricted cash			(36,929)	95,647
Cash and cash equivalents inclusive of restricted cash at end of period	$ 477,928	$ 600,296	$ 477,928	$ 600,296	$ 573,383	$ 515,646