CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Document Period End Date	Dec. 31, 2018
Cash, Cash Equivalents and Restricted Cash	$ 777,596	$ 824,088	$ 781,283	$ 673,173
Cash flows from operating activities:
Net income	344,255	399,630	257,491
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	86,544	85,446	86,663
Depreciation	22,834	22,698	21,003
Share-based Compensation	33,519	30,631	16,052
Change in estimated acquisition earn-out payables	2,969	9,200	9,185
Deferred income taxes	15,008	(102,183)	18,163
Amortization of debt discount and disposal of deferred financing costs	1,627	1,840	1,762
Accretion (Amortization) of Discounts and Premiums, Investments	(10)	(22)	(39)
Income tax benefit from exercise of shares from the stock benefit plans	0	0	(7,346)
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables	(12,538)	(14,501)	(3,904)
(Gain)/loss on sales of investments, fixed assets and customer accounts	(1,934)	(1,841)	596
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable (increase) decrease	(93,630)	(43,306)	(63,550)
Reinsurance recoverables (increase) decrease	412,424	(399,737)	(46,115)
Prepaid reinsurance premiums (increase) decrease	(16,903)	(12,356)	982
Other assets (increase) decrease	(22,440)	(9,747)	(4,718)
Premiums payable to insurance companies (increase) decrease	141,169	37,380	66,084
Premium deposits and credits due customers increase (decrease)	13,792	7,750	527
Losses and loss adjustment reserve increase (decrease)	(411,509)	398,638	46,115
Unearned premiums increase (decrease)	16,903	12,356	(982)
Accounts payable increase (decrease)	21,880	26,798	30,174
Accrued expenses and other liabilities increase (decrease)	22,801	25,509	8,670
Other liabilities increase (decrease)	(9,232)	(32,252)	(25,849)
Net cash provided by operating activities	567,529	441,975	411,042
Cash flows from investing activities:
Additions to fixed assets	(41,520)	(24,192)	(17,765)
Payments for businesses acquired, net of cash acquired	(923,874)	(41,471)	(122,622)
Proceeds from sales of fixed assets and customer accounts	4,984	4,094	4,957
Purchases of investments	(9,284)	(10,665)	(25,872)
Proceeds from Sale, Maturity and Collection of Investments	17,923	9,644	18,890
Net cash used in investing activities	(951,771)	(62,590)	(142,412)
Cash flows from financing activities:
Payments on acquisition earn-outs	(14,059)	(29,265)	(24,309)
Proceeds from Issuance of Long-term Debt	300,000	0	0
Payments on long-term debt	(120,000)	(96,750)	(73,125)
Income tax benefit from exercise of shares from the stock benefit plans	0	0	7,346
Issuances of common stock for employee stock benefit plans	19,432	17,422	15,983
Repurchase of stock benefit plan shares for employees to fund tax withholdings	(12,155)	(7,565)	(8,495)
Purchase of treasury stock	(91,250)	(128,639)	(18,908)
Settlement (prepayment) of accelerated share repurchase program	(8,750)	(11,250)	(11,250)
Cash dividends paid	(84,690)	(77,712)	(70,262)
Net cash provided by (used in) financing activities	337,750	(336,580)	(160,520)
Payments of Debt Issuance Costs	(778)	(2,821)
Net increase (decrease) in cash and cash equivalents inclusive of restricted cash	(46,492)	42,805	108,110
Proceeds from Lines of Credit	600,000	0	0
Repayments of Lines of Credit	$ 250,000	$ 0	$ 0