Schedule I - Condensed Financial Information of Registrant (Condensed Cash Flow Statement) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net Cash Flows Provided by Operating Activities	$ 776,001	$ 95,839	$ 32,540
CASH FLOWS FROM INVESTING ACTIVITIES:
Sales of investments	441,430	696,534	1,350,003
Maturities of investments	98,901	93,225	135,394
Purchases of investments	(665,200)	(722,223)	(1,678,582)
Purchases of property and equipment	(62,010)	(59,525)	(73,101)
Sales (purchases) of restricted investments and other	2,027	(7,432)	5,466
Net cash (used in) provided by investing activities	(184,852)	579	(12,558)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net (decrease) increase in checks outstanding, net of deposits	0	(23,842)	23,842
Excess tax benefit on share-based compensation	2,230	620	6,089
Proceeds from exercise of stock options and employee stock purchases	27,727	10,762	16,941
Borrowings under financing arrangements	0	345,000	110,000
Repayments of borrowings under financing arrangements	0	(345,000)	(122,500)
Repurchase of common stock	(152,549)	(77,810)	(69,496)
Net cash (used in) provided by financing activities	(155,171)	(3,373)	89,875
Net (decrease) increase in cash and cash equivalents	435,978	93,045	109,857
Cash and cash equivalents, beginning of year	433,155	340,110	230,253
Cash and cash equivalents, end of year	869,133	433,155	340,110
SUPPLEMENTAL CASH FLOWS DISCLOSURE:
Interest paid	29,670	30,789	31,134
Income taxes paid	83,499	80,119	5,001
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Flows Provided by Operating Activities	153,063	111,385	165,141
CASH FLOWS FROM INVESTING ACTIVITIES:
Sales of investments	81,263	6,060	0
Maturities of investments	1,512	877	0
Purchases of investments	(3,032)	(6,841)	0
Purchases of property and equipment	(62,491)	(53,632)	(68,116)
Increase (decrease) in notes receivable from Subsidiaries	0	0	7,000
Capital contributions returned to Parent	0	2,300	1,500
Capital contributions to subsidiaries	(188,000)	(7,500)	(17,560)
Sales (purchases) of restricted investments and other	0	161	876
Net cash (used in) provided by investing activities	(170,748)	(58,575)	(76,300)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net (decrease) increase in checks outstanding, net of deposits	(46)	5	(44)
Excess tax benefit on share-based compensation	1,701	287	3,222
Net borrowings from subsidiaries	70,714	39,487	(9,103)
Proceeds from exercise of stock options and employee stock purchases	27,727	10,762	16,941
Borrowings under financing arrangements	0	345,000	110,000
Repayments of borrowings under financing arrangements	0	(345,000)	(122,500)
Repurchase of common stock	(152,549)	(77,810)	(69,496)
Net cash (used in) provided by financing activities	(52,453)	(27,269)	(70,980)
Net (decrease) increase in cash and cash equivalents	(70,138)	25,541	17,861
Cash and cash equivalents, beginning of year	131,350	105,809	87,948
Cash and cash equivalents, end of year	61,212	131,350	105,809
SUPPLEMENTAL CASH FLOWS DISCLOSURE:
Interest paid	29,670	30,789	31,134
Income taxes paid	$ 83,499	$ 80,119	$ 5,001