CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash and Cash Equivalents [Line Items]
Repayments of Lines of Credit	$ 350,000	$ 0
Document Period End Date	Mar. 31, 2019
Payments of Debt Issuance Costs	$ (3,698)	0
Cash, Cash Equivalents and Restricted Cash	624,887	826,377
Cash flows from operating activities:
Net income	113,896	90,828
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	26,192	20,539
Depreciation	6,035	5,552
Non-cash stock-based compensation	12,994	7,295
Change in estimated acquisition earn-out payables	1,210	2,466
Deferred income taxes	(14,875)	(12,093)
Amortization of debt discount	456	407
Accretion (Amortization) of Discounts and Premiums, Investments	(7)	3
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables	0	(715)
Net gain on sales of investments, fixed assets and customer accounts	799	(2,388)
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable decrease	(6,484)	7,667
Reinsurance recoverables decrease	14,040	329,771
Prepaid reinsurance premiums decrease	23,557	28,845
Other assets decrease	(29,521)	806
Premiums payable to insurance companies (decrease)	(61,229)	(2,691)
Premium deposits and credits due customers increase	(11,362)	5,910
Losses and loss adjustment reserve (decrease)	(13,856)	(329,874)
Unearned premiums (decrease)	(23,557)	(28,845)
Accounts payable increase	56,436	52,896
Accrued expenses and other liabilities (decrease)	(97,689)	(94,514)
Other liabilities (decrease)	8,321	(2,384)
Net cash provided by operating activities	5,356	79,481
Cash flows from investing activities:
Additions to fixed assets	(23,186)	(9,751)
Payments for businesses acquired, net of cash acquired	(95,081)	(33,576)
Proceeds from sales of fixed assets and customer accounts	32	142
Purchases of investments	(31)	(6,142)
Proceeds from Sale, Maturity and Collection of Investments	2,392	6,531
Net cash used in investing activities	(115,874)	(42,796)
Cash flows from financing activities:
Payments on acquisition earn-outs	(579)	(1,761)
Proceeds from Issuance of Long-term Debt	350,000	0
Payments on long-term debt	(8,750)	(5,000)
Issuances of common stock for employee stock benefit plans	16	720
Repurchase Of Stock Benefit Plan Shares For Employees To Fund Tax Withholdings	(6,832)	(7,659)
Purchase of treasury stock	0	(11,250)
Settlement (prepayment) of accelerated share repurchase program	0	11,250
Cash dividends paid	(22,348)	(20,696)
Net cash used in financing activities	(42,191)	(34,396)
Cash and cash equivalents inclusive of restricted cash at end of period	322,477	558,248
Cash and Cash Equivalents inclusive of Restricted Cash [Member]
Cash flows from financing activities:
Net increase in cash and cash equivalents inclusive of restricted cash	(152,709)	2,289
Cash and cash equivalents inclusive of restricted cash at end of period	$ 624,887	$ 826,377