Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Premiums receivable, allowance for doubtful accounts	$ 1,646	$ 1,671
Preferred stock, par value (usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (shares)	10,000,000	10,000,000
Preferred stock, shares issued (shares)	0	0
Preferred stock, shares outstanding (shares)	0	0
Common stock, par value (usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized (shares)	350,000,000	350,000,000
Common stock, shares issued (shares)	153,728,000	152,451,000
Treasury stock, shares (shares)	76,442,000	74,378,000
Current [Member]
Investments available for sale, amortized cost	$ 2,374,767	$ 1,777,404
Noncurrent [Member]
Investments available for sale, amortized cost	$ 7,637	$ 5,474