CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Document Period End Date			Sep. 30, 2017
Cash, Cash Equivalents and Restricted Cash			$ 823,207	$ 766,962	$ 781,283	$ 673,173
Cash flows from operating activities:
Net income	$ 75,913	$ 71,545	212,125	199,865
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	21,435	21,805	64,402	65,025
Depreciation	5,489	5,195	17,242	15,867
Non-cash stock-based compensation			22,362	11,593
Change in estimated acquisition earn-out payables	(1,308)	3,610	8,309	6,846
Deferred income taxes			23,941	20,081
Amortization of debt discount			119	118
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Amortization Expense			1,309	1,207
Accretion (Amortization) of Discounts and Premiums, Investments			20	36
Income tax benefit from exercise of shares from the stock benefit plans			0	(7,213)
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables			(13,800)	(3,683)
Net loss/(gain) on sales of investments, fixed assets and customer accounts			(1,739)	(2,860)
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable decrease (increase)			(23,350)	(31,324)
Reinsurance recoverables decrease (increase)			(2,082,203)	(300,036)
Prepaid reinsurance premiums decrease			(23,585)	(24,324)
Other assets (increase)			(5,314)	1,231
Premiums payable to insurance companies decrease			5,585	39,787
Premium deposits and credits due customers increase			14,030	27,914
Losses and loss adjustment reserve (decrease) increase			2,082,203	300,036
Unearned premiums (decrease)			23,585	24,324
Accounts payable increase			22,113	13,858
Accrued expenses and other liabilities (decrease)			1,018	(22,119)
Other liabilities (decrease)			(34,802)	(17,094)
Net cash provided by operating activities			313,570	319,135
Cash flows from investing activities:
Additions to fixed assets	(4,049)	(4,191)	(12,897)	(13,135)
Payments for businesses acquired, net of cash acquired			(26,478)	(113,219)
Proceeds from sales of fixed assets and customer accounts			4,085	3,411
Purchases of investments			(10,393)	(24,332)
Proceeds from Sale, Maturity and Collection of Investments			5,178	16,716
Net cash used in investing activities			(40,505)	(130,559)
Cash flows from financing activities:
Payments on acquisition earn-outs			(25,488)	(23,872)
Payments on long-term debt			(91,750)	(34,375)
Income tax benefit from exercise of shares from the stock benefit plans			0	7,213
Issuances of common stock for employee stock benefit plans			17,387	15,959
Repurchase Of Stock Benefit Plan Shares For Employees To Fund Tax Withholdings			(6,791)	(8,395)
Purchase of treasury stock			(57,389)	(11,250)
Settlement of accelerated share repurchase program			(7,500)	11,250
Cash dividends paid			(56,801)	(51,317)
Net cash used in financing activities			(231,141)	(94,787)
Payments of Debt Issuance Costs			(2,809)
Net increase in cash and cash equivalents inclusive of restricted cash			41,924	93,789
Cash and cash equivalents inclusive of restricted cash at end of period	$ 546,520	$ 488,683	$ 546,520	$ 488,683	$ 515,646	$ 443,420