FAIR VALUE MEASUREMENTS (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 1,531,678,000	$ 1,399,162,000	$ 1,744,983,000	$ 1,579,003,000
State and municipal bonds	1,186,935,000	1,253,789,000
US Treasury securities	115,024,000	78,931,000
Government-sponsored enterprise securities	79,285,000	73,532,000
Residential mortgage-backed securities	337,198,000	312,641,000
Commercial mortgage-backed securities	33,892,000	21,590,000
Asset-backed securities	22,572,000	23,626,000
Corporate debt and other securities	1,035,012,000	991,610,000
Total	4,341,596,000	4,154,881,000
Debt Instruments [Abstract]
Senior Notes, Carrying Value	1,590,000,000	1,590,000,000
Senior Notes, Fair Value	1,800,000,000	1,810,000,000
Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,398,584,000	1,218,046,000
State and municipal bonds	0	0
US Treasury securities	115,024,000	78,931,000
Government-sponsored enterprise securities	0	0
Residential mortgage-backed securities	0	0
Commercial mortgage-backed securities	0	0
Asset-backed securities	0	0
Corporate debt and other securities	0	0
Total	1,513,608,000	1,296,977,000
Significant Other Observable Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	133,094,000	181,116,000
State and municipal bonds	1,186,935,000	1,253,789,000
US Treasury securities	0	0
Government-sponsored enterprise securities	79,285,000	73,532,000
Residential mortgage-backed securities	337,198,000	312,641,000
Commercial mortgage-backed securities	33,892,000	21,590,000
Asset-backed securities	22,572,000	23,626,000
Corporate debt and other securities	1,035,012,000	991,610,000
Total	2,827,988,000	2,857,904,000
Significant Unobservable Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
State and municipal bonds	0	0
US Treasury securities	0	0
Government-sponsored enterprise securities	0	0
Residential mortgage-backed securities	0	0
Commercial mortgage-backed securities	0	0
Asset-backed securities	0	0
Corporate debt and other securities	0	0
Total	$ 0	$ 0