Share-Based Compensation and Benefit Plans (Nonqualified Deferred Compensation Plan) (Narrative) (Details) - Nonqualified deferred compensation plan [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Share-Based Compensation and Benefit Plans
Deferred compensation plan, description			The Company sponsors a nonqualified deferred compensation plan (the “Deferred Compensation Plan”) for highly compensated employees whose contributions to the 401(k) Plan are limited due to the application of the annual limitations under the Internal Revenue Code. The Deferred Compensation Plan provides these employees with the opportunity to defer the full 6% of matched compensation, including salary and incentive based compensation that was precluded under the Company’s 401(k) Plan, which is then matched by the Company using the same formula as the 401(k) Plan.
Deferred compensation plan, obligation	$ 27.1		$ 27.1		$ 25.7
Deferred compensation plan, cost recognized	$ 0.1	$ 0.1	$ 0.1	$ 0.1