Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Premiums receivable, allowance for doubtful accounts	$ 3,731	$ 1,671
Preferred stock, par value (usd per share)	$ 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 (usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized	350,000,000	350,000,000
Common stock, shares issued	153,776,000	152,451,000
Treasury stock, shares	76,458,000	74,378,000
Current [Member]
Investments available for sale, amortized cost	$ 2,209,670	$ 1,777,404
Noncurrent [Member]
Investments available for sale, amortized cost	$ 34,197	$ 5,474