Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Denver Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring costs, percentage of reduction	6.00%
Restructuring costs, costs incurred	$ 1.9
Denver Restructuring [Member] | Employee Relocation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring costs, costs incurred	1.9
Tulsa Office Closure [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring costs, costs incurred		$ 8.3
Tulsa Office Closure [Member] | Employee Relocation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring costs, costs incurred	$ 0.6	3.0
Tulsa Office Closure [Member] | Special Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring costs, costs incurred		$ 5.3