CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Document Period End Date			Jun. 30, 2017
Cash, Cash Equivalents and Restricted Cash			$ 876,930	$ 731,585	$ 781,283	$ 673,173
Cash flows from operating activities:
Net income	$ 66,102	$ 66,250	136,212	128,320
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	21,347	21,610	42,967	43,220
Depreciation	5,655	5,354	11,753	10,672
Non-cash stock-based compensation			15,326	6,674
Change in estimated acquisition earn-out payables	5,589	4,057	9,617	3,236
Deferred income taxes			14,793	15,907
Amortization of debt discount			79	79
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Amortization Expense			943	807
Accretion (Amortization) of Discounts and Premiums, Investments			13	47
Income tax benefit from exercise of shares from the stock benefit plans			0	(6,816)
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables			(7,109)	(3,550)
Net loss/(gain) on sales of investments, fixed assets and customer accounts			136	(2,700)
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable decrease (increase)			2,918	(34,099)
Reinsurance recoverables decrease (increase)			55,604	(28,844)
Prepaid reinsurance premiums decrease			10,925	10,736
Other assets (increase)			(9,326)	(11,272)
Premiums payable to insurance companies decrease			59,219	90,068
Premium deposits and credits due customers increase			8,500	12,049
Losses and loss adjustment reserve (decrease) increase			(55,604)	28,844
Unearned premiums (decrease)			(10,925)	(10,736)
Accounts payable increase			38,510	34,156
Accrued expenses and other liabilities (decrease)			(26,021)	(29,765)
Other liabilities (decrease)			(28,449)	(18,346)
Net cash provided by operating activities			270,081	238,687
Cash flows from investing activities:
Additions to fixed assets	(5,816)	(4,692)	(8,848)	(8,944)
Payments for businesses acquired, net of cash acquired			(11,526)	(107,290)
Proceeds from sales of fixed assets and customer accounts			669	3,291
Purchases of investments			(5,916)	(16,405)
Proceeds from Sale, Maturity and Collection of Investments			2,911	9,379
Net cash used in investing activities			(22,710)	(119,969)
Cash flows from financing activities:
Payments on acquisition earn-outs			(8,668)	(7,012)
Payments on long-term debt			(86,750)	(20,625)
Income tax benefit from exercise of shares from the stock benefit plans			0	6,816
Issuances of common stock for employee stock benefit plans			500	916
Repurchase Of Stock Benefit Plan Shares For Employees To Fund Tax Withholdings			(5,054)	(6,273)
Purchase of treasury stock			(11,159)	(11,250)
Settlement of accelerated share repurchase program			0	11,250
Cash dividends paid			(37,840)	(34,128)
Net cash used in financing activities			(151,724)	(60,306)
Payments of Debt Issuance Costs			(2,753)
Net increase in cash and cash equivalents inclusive of restricted cash			95,647	58,412
Cash and cash equivalents inclusive of restricted cash at end of period	$ 600,296	$ 453,939	$ 600,296	$ 453,939	$ 515,646	$ 443,420