Incentive Award Plan and Stock Option Plan
 3 Months Ended
Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]  
Incentive Award Plan and Stock Option Plan
Incentive Award Plan
As of March 31, 2015, 2.0 million shares remained available for award under the Incentive Award Plan.
Restricted Shares of Common Stock
During the three months ended March 31, 2015, the Company granted 0.4 million restricted shares under the Incentive Award Plan to certain officers and employees. The Company recorded $4.3 million in deferred compensation associated with these grants, which will be recognized in expense over the service period of the awards. As of March 31, 2015, there were approximately 1.3 million unvested restricted shares outstanding.

Performance Share Awards
During the three months ended March 31, 2015, the Compensation Committee of the Board of Directors approved an initial target grant of 236,826 performance shares to certain executive officers of the Company. The performance period of this grant runs from January 1, 2015 through December 31, 2017. Pursuant to the performance share award agreement, each participant is eligible to vest in and receive shares of the Corporation's common stock based on the initial target number of shares granted multiplied by a percentage range between 0% and 250%. The percentage range is based on the attainment of TSR of the Corporation compared to certain specified peer groups of companies during the performance period. In addition, final shares issued under each performance share award entitle its holder to a cash payment equal to the aggregate dividends that were declared during the performance period as if the shares had been issued and outstanding on each dividend record date. Based on the grant date fair value, the Corporation expects to recognize $3.5 million in compensation expense on a straight-line basis over the requisite service period associated with this market-based grant.
As of March 31, 2015, under each separate annual performance share award, the Corporation's TSR compared to the specified peer groups during the performance periods would have resulted in the release of 1.0 million shares, in the aggregate. In addition, approximately $0.7 million in dividend rights have been accrued. The projected shares to be released are not considered issued under the Incentive Award Plan until the performance period has ended and the actual number of shares to be released is determined. The performance shares and dividend rights are subject to forfeiture in the event of a non-qualifying termination of a participant prior to the performance period end date.
Stock-based Compensation Expense
For the three months ended March 31, 2015 and 2014, the Company recognized $3.8 million and $2.5 million, respectively, in stock-based compensation expense, which is included in general and administrative expenses in the accompanying consolidated statements of operations.
As of March 31, 2015, the remaining unamortized stock-based compensation expense, including amounts relating to the performance share awards, totaled $16.2 million, which is recognized as the greater of the amount amortized on a straight-line basis over the service period of each applicable award or the amount vested over the vesting periods.