Incentive Plan (401(k)) (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Vesting percentage of discretionary contributions	100.00%
Aggregate cost of contributions to the 401(k) Plan		$ 4.6	$ 3.6	$ 3.4