Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Premiums receivable, allowance for doubtful accounts	$ 763	$ 668
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	350,000,000	350,000,000
Common stock, shares issued	150,193,000	148,727,000
Treasury stock, shares	70,696,000	67,426,000
Current [Member]
Investments available for sale, amortized cost	1,611,091	1,753,931
Noncurrent [Member]
Investments available for sale, amortized cost	$ 60,080	$ 0