401(k) Plan	12 Months Ended
Dec. 31, 2017
401(k) Plan [Abstract]
Compensation and Employee Benefit Plans [Text Block]	401(k) Plan We sponsor a defined contribution plan under Section 401(k) of the Internal Revenue Code for certain employees who have completed at least three months of service. Employees may elect to contribute up to 50% of their eligible compensation on a pre-tax basis, subject to limitations. We may make discretionary contributions to the 401(k) plan. Employer matching contributions are subject to a three-year graded vesting schedule based on the participant's years of service with us. For the years ended December 31, 2017, 2016 and 2015, we made matching cash contributions of $7.0 million, $7.6 million and $7.2 million, respectively, which represents 50% of the employees’ contributions to the 401(k) plan up to an amount not to exceed 6% of each employee's respective compensation. Employees are permitted to elect to purchase our common stock as part of their 401(k) plan. As of December 31, 2017 and 2016, 6.0% and 3.6%, respectively, of the total plan assets consisted of our common stock.

401(k) Plan

12 Months Ended

Dec. 31, 2017

Compensation and Employee Benefit Plans [Text Block]

401(k) Plan

We sponsor a defined contribution plan under Section 401(k) of the Internal Revenue Code for certain employees who have completed at least three months of service. Employees may elect to contribute up to 50% of their eligible compensation on a pre-tax basis, subject to limitations. We may make discretionary contributions to the 401(k) plan. Employer matching contributions are subject to a three-year graded vesting schedule based on the participant's years of service with us. For the years ended December 31, 2017, 2016 and 2015, we made matching cash contributions of $7.0 million, $7.6 million and $7.2 million, respectively, which represents 50% of the employees’ contributions to the 401(k) plan up to an amount not to exceed 6% of each employee's respective compensation. Employees are permitted to elect to purchase our common stock as part of their 401(k) plan. As of December 31, 2017 and 2016, 6.0% and 3.6%, respectively, of the total plan assets consisted of our common stock.