Schedule I - Condensed Financial Information of Registrant (Condensed Cash Flow Statement) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net Cash Flows Provided by Operating Activities	$ 431,095	$ 776,001	$ 95,839
CASH FLOWS FROM INVESTING ACTIVITIES:
Sales of investments	953,092	441,430	696,534
Maturities of investments	104,095	98,901	93,225
Purchases of investments	(1,538,128)	(665,200)	(722,223)
Purchases of property and equipment	(60,949)	(62,010)	(59,525)
Sales (purchases) of restricted investments and other	(4,759)	2,027	(7,432)
Net cash (used in) provided by investing activities	(546,649)	(184,852)	579
CASH FLOWS FROM FINANCING ACTIVITIES:
Net decrease in checks outstanding, net of deposits	0	0	(23,842)
Excess tax benefit on share-based compensation	4,855	2,230	620
Proceeds from exercise of stock options and employee stock purchases	18,972	27,727	10,762
Borrowings under financing arrangements	360,000	0	345,000
Repayments of borrowings under financing arrangements	(175,000)	0	(345,000)
Repurchase of common stock	(112,977)	(152,549)	(77,810)
Net cash provided by (used in) financing activities	243,275	(155,171)	(3,373)
Net (decrease) increase in cash and cash equivalents	127,721	435,978	93,045
Cash and cash equivalents, beginning of year	869,133	433,155	340,110
Cash and cash equivalents, end of year	996,854	869,133	433,155
SUPPLEMENTAL CASH FLOWS DISCLOSURE:
Interest paid	31,622	29,670	30,789
Income taxes paid	276,468	83,499	80,119
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Flows Provided by Operating Activities	323,057	153,063	111,385
CASH FLOWS FROM INVESTING ACTIVITIES:
Sales of investments	0	81,263	6,060
Maturities of investments	0	1,512	877
Purchases of investments	0	(3,032)	(6,841)
Purchases of property and equipment	(57,749)	(62,491)	(53,632)
Capital contributions returned to Parent	518	0	2,300
Capital contributions to subsidiaries	(414,200)	(188,000)	(7,500)
Sales (purchases) of restricted investments and other	0	0	161
Net cash (used in) provided by investing activities	(471,431)	(170,748)	(58,575)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net decrease in checks outstanding, net of deposits	38	(46)	5
Excess tax benefit on share-based compensation	2,915	1,701	287
Net borrowings from subsidiaries	21,277	70,714	39,487
Proceeds from exercise of stock options and employee stock purchases	18,972	27,727	10,762
Borrowings under financing arrangements	360,000	0	345,000
Repayments of borrowings under financing arrangements	(175,000)	0	(345,000)
Repurchase of common stock	(112,977)	(152,549)	(77,810)
Net cash provided by (used in) financing activities	115,225	(52,453)	(27,269)
Net (decrease) increase in cash and cash equivalents	(33,149)	(70,138)	25,541
Cash and cash equivalents, beginning of year	61,212	131,350	105,809
Cash and cash equivalents, end of year	28,063	61,212	131,350
SUPPLEMENTAL CASH FLOWS DISCLOSURE:
Interest paid	31,622	29,670	30,789
Income taxes paid	$ 276,468	$ 83,499	$ 80,119