Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Premiums receivable, allowance for doubtful accounts	$ 2,504	$ 1,671
Preferred stock, par value (usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (shares)	10,000,000us-gaap_PreferredStockSharesAuthorized	10,000,000us-gaap_PreferredStockSharesAuthorized
Preferred stock, shares issued (shares)	0us-gaap_PreferredStockSharesIssued	0us-gaap_PreferredStockSharesIssued
Preferred stock, shares outstanding (shares)	0us-gaap_PreferredStockSharesOutstanding	0us-gaap_PreferredStockSharesOutstanding
Common stock, par value (usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized (shares)	350,000,000us-gaap_CommonStockSharesAuthorized	350,000,000us-gaap_CommonStockSharesAuthorized
Common stock, shares issued (shares)	153,483,000us-gaap_CommonStockSharesIssued	152,451,000us-gaap_CommonStockSharesIssued
Treasury stock, shares (shares)	76,434,000us-gaap_TreasuryStockShares	74,378,000us-gaap_TreasuryStockShares
Current [Member]
Investments available for sale, amortized cost	2,490,857	1,777,404
Noncurrent [Member]
Investments available for sale, amortized cost	$ 7,471	$ 5,474

A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

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