CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Cash, Cash Equivalents and Restricted Cash	$ 826,089	$ 632,921
Cash flows from operating activities:
Net income	70,110	62,070
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization	21,620	21,610
Depreciation	6,098	5,318
Non-cash stock-based compensation	8,564	2,772
Change in estimated acquisition earn-out payables	4,028	(821)
Deferred income taxes	10,005	9,933
Amortization of debt discount	39	39
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Amortization Expense	383	420
Accretion (Amortization) of Discounts and Premiums, Investments	8	18
Income tax benefit from exercise of shares from the stock benefit plans	0	(1,249)
Payments On Acquisition Earn Outs In Excess Of Original Estimated Payables	(4,547)	(3,550)
Net gain on sales of investments, fixed assets and customer accounts	(2)	(1,911)
Changes in operating assets and liabilities, net of effect from acquisitions and divestitures:
Premiums, commissions and fees receivable decrease	16,297	5,229
Reinsurance recoverables decrease (increase)	38,844	(26,161)
Prepaid reinsurance premiums decrease	28,521	29,859
Other assets decrease (increase)	1,404	(21,247)
Premiums payable to insurance companies (decrease)	(4,383)	(21,139)
Premium deposits and credits due customers increase	7,088	6,238
Losses and loss adjustment reserve (decrease) increase	(38,844)	26,161
Unearned premiums (decrease)	(28,521)	(29,859)
Accounts payable increase	45,386	22,962
Accrued expenses and other liabilities (decrease)	(77,325)	(50,034)
Other liabilities (decrease)	(14,752)	(3,436)
Net cash provided by operating activities	90,021	33,222
Cash flows from investing activities:
Additions to fixed assets	(3,032)	(4,252)
Payments for businesses acquired, net of cash acquired	0	(40,603)
Proceeds from sales of fixed assets and customer accounts	589	2,467
Purchases of investments	(379)	(10,106)
Proceeds from Sale, Maturity and Collection of Investments	679	7,604
Net cash used in investing activities	(2,143)	(44,890)
Cash flows from financing activities:
Payments on acquisition earn-outs	(5,683)	(5,527)
Proceeds from long-term debt	(14,250)	(6,875)
Income tax benefit from exercise of shares from the stock benefit plans	0	1,249
Issuances of common stock for employee stock benefit plans	506	916
Repurchase Of Stock Benefit Plan Shares For Employees To Fund Tax Withholdings	(4,742)	(1,385)
Purchase of treasury stock	0	(11,250)
Settlement of accelerated share repurchase program	0	11,250
Cash dividends paid	(18,903)	(16,962)
Net cash used in financing activities	(43,072)	(28,584)
Net increase (decrease) in cash and cash equivalents inclusive of restricted cash	44,806	(40,252)
Cash, Cash Equivalents and Restricted Cash- Beginning Balance			$ 781,283	$ 673,173
Cash and cash equivalents inclusive of restricted cash at beginning of period	546,721	401,846	515,646	443,420
Cash and Cash Equivalents inclusive of Restricted Cash [Member]
Cash flows from financing activities:
Cash and cash equivalents inclusive of restricted cash at beginning of period	$ 826,089	$ 632,921	$ 781,283	$ 673,173