CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash and Cash Equivalents [Line Items]
Document Period End Date	Mar. 31, 2018
Cash, Cash Equivalents and Restricted Cash	$ 826,377	$ 826,089
Cash flows from operating activities:
Net income	90,828	70,110
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	20,539	21,620
Depreciation	5,552	6,098
Non-cash stock-based compensation	7,295	8,564
Change in estimated acquisition earn-out payables	2,466	4,028
Deferred income taxes	(12,093)	10,005
Amortization of debt discount	39	39
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Amortization Expense	368	383
Accretion (Amortization) of Discounts and Premiums, Investments	3	8
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables	(715)	(4,547)
Net gain on sales of investments, fixed assets and customer accounts	(2,388)	(2)
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable decrease	7,667	16,297
Reinsurance recoverables decrease	329,771	38,844
Prepaid reinsurance premiums decrease	28,845	28,521
Other assets decrease	806	1,404
Premiums payable to insurance companies (decrease)	(2,691)	(4,383)
Premium deposits and credits due customers increase	5,910	7,088
Losses and loss adjustment reserve (decrease)	(329,874)	(38,844)
Unearned premiums (decrease)	(28,845)	(28,521)
Accounts payable increase	52,896	45,386
Accrued expenses and other liabilities (decrease)	(94,514)	(77,325)
Other liabilities (decrease)	(2,384)	(14,752)
Net cash provided by operating activities	79,481	90,021
Cash flows from investing activities:
Additions to fixed assets	(9,751)	(3,032)
Payments for businesses acquired, net of cash acquired	(33,576)	0
Proceeds from sales of fixed assets and customer accounts	142	589
Purchases of investments	(6,142)	(379)
Proceeds from Sale, Maturity and Collection of Investments	6,531	679
Net cash used in investing activities	(42,796)	(2,143)
Cash flows from financing activities:
Payments on acquisition earn-outs	(1,761)	(5,683)
Payments on long-term debt	(5,000)	(14,250)
Issuances of common stock for employee stock benefit plans	720	506
Repurchase Of Stock Benefit Plan Shares For Employees To Fund Tax Withholdings	(7,659)	(4,742)
Purchase of treasury stock	(11,250)	0
Settlement (prepayment) of accelerated share repurchase program	11,250	0
Cash dividends paid	(20,696)	(18,903)
Net cash used in financing activities	(34,396)	(43,072)
Net increase in cash and cash equivalents inclusive of restricted cash	2,289	44,806
Cash and cash equivalents inclusive of restricted cash at end of period	$ 558,248	$ 546,721