Defined Contribution Benefit Plan Defined Contribution Benefit Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Benefit Plan [Line Items]
Maximum annual employee contribution percentage	100.00%
Defined contribution plan cost	$ 6.2	$ 5.1	$ 4.6
Employer Match on Employee Contributions up to Three Percent of Employee Salary [Member]
Defined Contribution Benefit Plan [Line Items]
Employer match percentage	100.00%
Employer Match on Employee Contributions Between Three and Five Percent of Employee Salary [Member]
Defined Contribution Benefit Plan [Line Items]
Employer match percentage	50.00%
Maximum Employee Contribution Percentage Eligible for 100 Percent Employer Match [Member]
Defined Contribution Benefit Plan [Line Items]
Maximum annual employee contribution percentage	3.00%
Maximum Employee Contribution Percentage Eligible for 50 Percent Employer Match After 100 Percent Employer Match [Member]
Defined Contribution Benefit Plan [Line Items]
Maximum annual employee contribution percentage	2.00%