CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Document Period End Date			Sep. 30, 2018
Payments of Debt Issuance Costs			$ 0	$ (2,809)
Cash, Cash Equivalents and Restricted Cash			724,687	823,207	$ 824,088	$ 781,283
Cash flows from operating activities:
Net income	$ 106,053	$ 75,913	270,803	212,125
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	21,637	21,435	62,961	64,402
Depreciation	5,259	5,489	16,410	17,242
Non-cash stock-based compensation			23,522	22,362
Change in estimated acquisition earn-out payables	(357)	(1,308)	2,528	8,309
Deferred income taxes			(16,416)	23,941
Amortization of debt discount			118	119
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Amortization Expense			1,102	1,309
Accretion (Amortization) of Discounts and Premiums, Investments			(4)	20
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables			(11,775)	(13,800)
Net gain on sales of investments, fixed assets and customer accounts			(1,259)	(1,739)
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable decrease			(24,560)	(23,350)
Reinsurance recoverables decrease			379,304	(2,082,203)
Prepaid reinsurance premiums decrease			(23,938)	(23,585)
Other assets decrease			(9,052)	(5,314)
Premiums payable to insurance companies (decrease)			19,179	5,585
Premium deposits and credits due customers increase			16,089	14,030
Losses and loss adjustment reserve (decrease)			(378,246)	2,082,203
Unearned premiums (decrease)			23,938	23,585
Accounts payable increase			27,817	22,113
Accrued expenses and other liabilities (decrease)			(21,465)	1,018
Other liabilities (decrease)			(1,539)	(34,802)
Net cash provided by operating activities			355,517	313,570
Cash flows from investing activities:
Additions to fixed assets	(9,469)	(4,049)	(28,859)	(12,897)
Payments for businesses acquired, net of cash acquired			(254,836)	(26,478)
Proceeds from sales of fixed assets and customer accounts			3,338	4,085
Purchases of investments			(8,897)	(10,393)
Proceeds from Sale, Maturity and Collection of Investments			17,551	5,178
Net cash used in investing activities			(271,703)	(40,505)
Cash flows from financing activities:
Payments on acquisition earn-outs			(13,337)	(25,488)
Payments on long-term debt			(115,000)	(91,750)
Issuances of common stock for employee stock benefit plans			19,406	17,387
Repurchase Of Stock Benefit Plan Shares For Employees To Fund Tax Withholdings			(11,928)	(6,791)
Purchase of treasury stock			(11,250)	(57,389)
Settlement (prepayment) of accelerated share repurchase program			11,250	(7,500)
Cash dividends paid			(62,356)	(56,801)
Net cash used in financing activities			(183,215)	(231,141)
Net increase in cash and cash equivalents inclusive of restricted cash			(99,401)	41,924
Cash and cash equivalents inclusive of restricted cash at end of period	$ 422,971	$ 546,520	$ 422,971	$ 546,520	$ 573,383	$ 515,646