Approximately 38,500 independent insurance agencies are estimated to be operating currently in the United States. Part of our continuing business strategy is to attract high-quality insurance intermediaries to join our operations. From 1993 through 2013, we acquired 449 insurance intermediary operations, excluding acquired books of business (customer accounts).
We have acquired significant intangible assets through business acquisitions. These assets consist of purchased customer accounts, non-compete agreements, and the excess of purchase prices over the fair value of identifiable net assets acquired (Goodwill). The determination of estimated useful lives and the allocation of purchase price to intangible assets requires significant judgment and affects the amount of future amortization and possible impairment charges.
We have an employee stock purchase plan, and grant stock options and non-vested stock awards to our employees. Compensation expense for all share-based awards is recognized in the financial statements based upon the grant-date fair value of those awards. For 2013, 2012 and 2011, the non-cash stock-based compensation expense incorporates the costs related to each of our four stock-based plans as explained in Note 11 of the Notes to the Consolidated Financial Statements.
As of December 31, 2013, the estimated acquisition earn-out payables equaled $43,058,000, of which $6,312,000 was recorded as accounts payable and $36,746,000 was recorded as other non-current liability. As of December 31, 2012, the estimated acquisition earn-out payables equaled $52,987,000, of which $10,164,000 was recorded as accounts payable and $42,823,000 was recorded as other non-current liability.
On December 22, 2006, we entered into a Master Shelf and Note Purchase Agreement (the “Master Agreement”) with a national insurance company (the “Purchaser”). On September 30, 2009, we and the Purchaser amended the Master Agreement to extend the term of the agreement until August 20, 2012. The Purchaser also purchased Notes issued by us in 2004. The Master Agreement provides for a $200.0 million private uncommitted “shelf” facility for the issuance of senior unsecured notes over a three-year period, with interest rates that may be fixed or floating and with such maturity dates, not to exceed ten years, as the parties may determine. The Master Agreement includes various covenants, limitations and events of default similar to the Notes issued in 2004. The initial issuance of notes under the Master Agreement occurred on December 22, 2006, through the issuance of $25.0 million in Series C Senior Notes due December 22, 2016, with a fixed interest rate of 5.66% per year. On February 1, 2008, $25.0 million in Series D Senior Notes due January 15, 2015, with a fixed interest rate of 5.37% per year, were issued. On September 15, 2011, and pursuant to a Confirmation of Acceptance, dated January 21, 2011 (the “Confirmation”), in connection with the Master Agreement, $100.0 million in Series E Senior Notes due September 15, 2018, with a fixed interest rate of 4.50% per year, were issued. The Series E Senior Notes were issued for the sole purpose of retiring the Series A Senior Notes. As of December 31, 2013 and December 31, 2012, there was an outstanding debt balance of $150.0 million attributable to notes issued under the provisions of the Master Agreement. The Master Agreement expired on September 30, 2012 and was not extended.
Income taxes paid in 2013, 2012 and 2011 were $110,191,000, $80,622,000, and $75,403,000, respectively.
For the five months ended December 31, 2013, 2012 and 2011 (portions of the 2013-2014, 2012-2013 and 2011-2012 plan years), 222,526, 246,164, and 230,481 shares of common stock (from authorized but unissued shares), respectively, were subscribed to by ESPP participants for proceeds of approximately $5,937,000, $5,278,000 and $3,810,000, respectively.