CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Cash Flows [Abstract]
Cash, Cash Equivalents and Restricted Cash	$ 824,088	$ 781,283	$ 673,173	$ 729,817
Cash flows from operating activities:
Net income	399,630	257,491	243,318
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	85,446	86,663	87,421
Depreciation	22,698	21,003	20,890
Share-based Compensation	30,631	16,052	15,513
Change in estimated acquisition earn-out payables	9,200	9,185	3,003
Deferred income taxes	(102,183)	18,163	22,696
Amortization of debt discount	158	165	157
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Amortization Expense	1,682	1,597	0
Accretion (Amortization) of Discounts and Premiums, Investments	22	(39)	0
Income tax benefit from exercise of shares from the stock benefit plans	0	(7,346)	(3,276)
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables	(14,501)	(3,904)	(11,383)
Loss/(gain) on sales of investments, fixed assets and customer accounts	(1,841)	596	(107)
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable (increase)	(43,306)	(63,550)	(7,163)
Reinsurance recoverables (increase)	(399,737)	(46,115)	(18,940)
Prepaid reinsurance premiums (increase) decrease	(12,356)	982	10,943
Other assets (increase)	(9,747)	(4,718)	(5,318)
Premiums payable to insurance companies decrease	37,380	66,084	542
Premium deposits and credits due customers increase (decrease)	7,750	527	(2,973)
Losses and loss adjustment reserve increase	398,638	46,115	18,940
Unearned premiums increase (decrease)	12,356	(982)	(10,943)
Accounts payable increase	26,798	30,174	34,206
Accrued expenses and other liabilities increase	25,509	8,670	8,204
Other liabilities (decrease)	(32,252)	(25,849)	(23,898)
Net cash provided by operating activities	441,975	411,042	381,832
Cash flows from investing activities:
Additions to fixed assets	(24,192)	(17,765)	(18,375)
Payments for businesses acquired, net of cash acquired	(41,471)	(122,622)	(136,000)
Proceeds from sales of fixed assets and customer accounts	4,094	4,957	10,576
Purchases of investments	(10,665)	(25,872)	(22,766)
Proceeds from Sale, Maturity and Collection of Investments	9,644	18,890	21,928
Net cash used in investing activities	(62,590)	(142,412)	(144,637)
Cash flows from financing activities:
Payments on acquisition earn-outs	(29,265)	(24,309)	(25,415)
Payments on long-term debt	(96,750)	(73,125)	(45,625)
Income tax benefit from exercise of shares from the stock benefit plans	0	7,346	3,276
Issuances of common stock for employee stock benefit plans	17,422	15,983	15,890
Repurchase of stock benefit plan shares for employees to fund tax withholdings	(7,565)	(8,495)	(2,857)
Purchase of treasury stock	(128,639)	(18,908)	(163,750)
Settlement (prepayment) of accelerated share repurchase program	(11,250)	11,250	11,250
Cash dividends paid	(77,712)	(70,262)	(64,108)
Net cash used in by financing activities	(336,580)	(160,520)	(293,839)
Payments of Debt Issuance Costs	(2,821)
Net increase (decrease) in cash and cash equivalents inclusive of restricted cash	$ 42,805	$ 108,110	$ (56,644)