CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 257,491	$ 243,318	$ 206,896
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	86,663	87,421	82,941
Depreciation	21,003	20,890	20,895
Share-based Compensation	16,052	15,513	19,363
Change in estimated acquisition earn-out payables	9,185	3,003	9,938
Deferred income taxes	18,163	22,696	7,369
Amortization of debt discount	165	157	46
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Amortization Expense	1,597	0	0
Accretion (Amortization) of Discounts and Premiums, Investments	39	0	0
Income tax benefit from exercise of shares from the stock benefit plans	(7,346)	(3,276)	(3,298)
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables	(3,904)	(11,383)	(2,539)
Loss/(gain) on sales of investments, fixed assets and customer accounts	596	(107)	42,465
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Restricted cash and investments (increase) decrease	(35,884)	30,016	(9,760)
Premiums, commissions and fees receivable (increase)	(63,550)	(7,163)	(11,160)
Reinsurance recoverables (increase) decrease	(46,115)	(18,940)	12,210
Prepaid reinsurance premiums decrease (increase)	982	10,943	(31,573)
Other assets (increase)	(4,718)	(5,318)	(12,564)
Premiums payable to insurance companies decrease	66,084	542	8,164
Premium deposits and credits due customers increase (decrease)	527	(2,973)	2,323
Losses and loss adjustment reserve increase (decrease)	46,115	18,940	(12,210)
Unearned premiums (decrease) increase	(982)	(10,943)	31,573
Accounts payable increase	30,174	34,206	36,949
Accrued expenses and other liabilities increase	8,670	8,204	11,718
Other liabilities (decrease)	(25,849)	(23,898)	(24,727)
Net cash provided by operating activities	375,158	411,848	385,019
Cash flows from investing activities:
Additions to fixed assets	(17,765)	(18,375)	(24,923)
Payments for businesses acquired, net of cash acquired	(122,622)	(136,000)	(696,486)
Proceeds from sales of fixed assets and customer accounts	4,957	10,576	13,631
Purchases of investments	(25,872)	(22,766)	(17,813)
Proceeds from Sale, Maturity and Collection of Investments	18,890	21,928	18,278
Net cash used in investing activities	(142,412)	(144,637)	(707,313)
Cash flows from financing activities:
Payments on acquisition earn-outs	(24,309)	(25,415)	(9,530)
Proceeds from Issuance of Long-term Debt	0	0	1,048,425
Payments on long-term debt	(73,125)	(45,625)	(330,000)
Income tax benefit from exercise of shares from the stock benefit plans	7,346	3,276	3,298
Issuances of common stock for employee stock benefit plans	15,983	15,890	14,808
Repurchase of stock benefit plan shares for employees to fund tax withholdings	(8,495)	(2,857)	(3,252)
Purchase of treasury stock	(18,908)	(163,750)	(75,025)
Settlement (prepayment) of accelerated share repurchase program	11,250	(11,250)	0
Cash dividends paid	(70,262)	(64,108)	(59,334)
Net cash (used in) provided by financing activities	(160,520)	(293,839)	589,390
Net increase (decrease) in cash and cash equivalents	72,226	(26,628)	267,096
Cash and cash equivalents at beginning of period	443,420	470,048
Cash and cash equivalents at end of period	515,646	443,420	470,048
Proceeds from Lines of Credit	0	0	475,000
Repayments of Lines of Credit	$ 0	$ 0	$ (475,000)