Sch II - Condensed Financial Information on Registrant Cash Flow (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	$ 239,794	$ 224,910	$ 208,758
Equity securities - available for sale	(11,057)	(23,833)	(19,178)
Short-term investments, net change	11,903	20,334	60,092
Net Cash Provided by (Used in) Investing Activities	(387,516)	(369,357)	(308,643)
Repayments of debt	(15,000)	0	0
Excess tax deductions on stock-based compensation	846	1,362	1,199
Repurchase of common stock, net	1,840	(584)	(8,003)
Dividends paid	(91,602)	(89,347)	(34,749)
Net Cash Provided by (Used in) Financing Activities	151,815	97,305	170,147
Increase in cash and cash equivalents	4,093	(47,142)	70,262
Cash and cash equivalents at end of period	33,583	29,490	76,632
Cash and cash equivalents at beginning of period	29,490	76,632	6,370
Income Taxes Received, Net	20,894	27,701	22,802
Cash paid during the period for interest	4,854	4,850	4,850
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	(4,342)	1,841	4,200
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	5,641	18,618	21,347
Equity securities - available for sale	(1,397)	(1,188)	(269)
Short-term investments, net change	7,078	395	17,617
Dividends from subsidiaries (eliminated in consolidation)	85,900	50,000	45,700
Net Cash Provided by (Used in) Investing Activities	97,222	67,825	84,395
Excess tax deductions on stock-based compensation	846	1,362	1,199
Repurchase of common stock, net	1,840	(584)	(8,003)
Capital contribution by parent to subsidiary	0	(300)	(1,000)
Dividends paid	(91,602)	(89,347)	(34,749)
Net Cash Provided by (Used in) Financing Activities	(88,916)	(88,869)	(42,553)
Increase in cash and cash equivalents	3,964	(19,203)	46,042
Cash and cash equivalents at end of period	30,803	26,839	46,042
Cash and cash equivalents at beginning of period	26,839	46,042	0
Income Taxes Received, Net	(5,486)	(6,344)	6,927
Cash paid during the period for interest	4,850	4,850	4,850
Dividends in form of cash [Member] | Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividends from subsidiaries (eliminated in consolidation)	$ 85,900	$ 50,000	$ 45,700