Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Premiums receivable, allowance for doubtful accounts	$ 772	$ 643
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	151,442,000	150,224,000
Treasury stock, shares	71,242,000	70,704,000
Current [Member]
Investments available for sale, amortized cost	1,699,933	1,602,456
Noncurrent [Member]
Investments available for sale, amortized cost	$ 4,861	$ 67,943