CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Repayments of Lines of Credit					$ 350,000	$ 0
Document Period End Date					Jun. 30, 2019
Payments of Debt Issuance Costs					$ (3,701)	0
Cash, Cash Equivalents and Restricted Cash					814,781	787,159	$ 777,596	$ 824,088
Cash flows from operating activities:
Net income	$ 92,593	$ 113,896	$ 73,922	$ 90,828	206,489	164,750
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	25,954		20,785		52,146	41,324
Depreciation	5,666		5,599		11,701	11,151
Non-cash stock-based compensation					24,128	15,027
Change in estimated acquisition earn-out payables	(2,860)		419		(1,650)	2,885
Deferred income taxes					(5,342)	(16,437)
Amortization of debt discount and disposal of deferred financing costs					988	819
Accretion (Amortization) of Discounts and Premiums, Investments					(12)	1
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables					(267)	(3,408)
Net gain/(loss) on sales of investments, fixed assets and customer accounts					(346)	(2,432)
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable (increase) decrease					(71,614)	(29,471)
Reinsurance recoverables (increase) decrease					(11,939)	388,704
Prepaid reinsurance premiums (increase) decrease					(5,674)	10,310
Other assets (increase) decrease					(22,122)	(15,731)
Premiums payable to insurance companies increase (decrease)					116,472	106,839
Premium deposits and credits due customers increase (decrease)					(12,003)	8,360
Losses and loss adjustment reserve increase (decrease)					12,123	(388,572)
Unearned premiums increase (decrease)					5,674	(10,310)
Accounts payable increase (decrease)					23,773	31,343
Accrued expenses and other liabilities increase (decrease)					(32,269)	(34,837)
Other liabilities increase (decrease)					15,447	(2,909)
Net cash provided by operating activities					305,703	277,406
Cash flows from investing activities:
Additions to fixed assets	(11,988)		(9,639)		(35,174)	(19,390)
Payments for businesses acquired, net of cash acquired					(146,646)	(141,803)
Proceeds from sales of fixed assets and customer accounts					2,101	2,906
Purchases of investments					(9,265)	(8,863)
Proceeds from Sale, Maturity and Collection of Investments					4,505	16,346
Net cash used in investing activities					(184,479)	(150,804)
Cash flows from financing activities:
Payments on acquisition earn-outs					(6,997)	(5,183)
Proceeds from Issuance of Long-term Debt					350,000	0
Payments on long-term debt					(22,500)	(110,001)
Issuances of common stock for employee stock benefit plans					901	720
Repurchase Of Stock Benefit Plan Shares For Employees To Fund Tax Withholdings					(6,861)	(7,656)
Purchase of treasury stock					(20,000)	(11,250)
Settlement (prepayment) of accelerated share repurchase program					20,000	11,250
Cash dividends paid					(44,881)	(41,411)
Net cash used in financing activities					(84,039)	(163,531)
Cash and cash equivalents inclusive of restricted cash at end of period	432,548		477,928		432,548	477,928	438,961	573,383
Cash and Cash Equivalents inclusive of Restricted Cash [Member]
Cash flows from financing activities:
Net increase (decrease) in cash and cash equivalents inclusive of restricted cash					37,185	(36,929)
Cash and cash equivalents inclusive of restricted cash at end of period	$ 814,781		$ 787,159		$ 814,781	$ 787,159	$ 777,596	$ 824,088