Stock-Based Compensation (Schedule of Fair Value of the Restricted Stock Awards Granted During the Periods) (Details) - Restricted Stock [Member] - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017
Time Vested [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		888,810	524,903
Estimated fair value (in millions)	[1]	$ 17.1	$ 12.7
Time Vested [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		844,498	475,799
Estimated fair value (in millions)	[1]	$ 16.2	$ 11.8
Percentage of shares granted expected to be distributed		95.00%	95.00%
Time Vested [Member] | Non-employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		44,312	49,104
Estimated fair value (in millions)	[1]	$ 0.9	$ 0.9
Percentage of shares granted expected to be distributed		100.00%	100.00%
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		362,070	173,373
Estimated fair value (in millions)	[1]	$ 7.3	$ 4.5
Performance Shares [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		362,070	173,373
Estimated fair value (in millions)	[1]	$ 7.3	$ 4.5
Percentage of shares granted expected to be distributed		74.00%	91.00%
Performance Shares [Member] | Non-employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		0	0
Estimated fair value (in millions)	[1]	$ 0.0	$ 0.0

[1] The performance shares represent 100% of the grant date fair value. (We recognize the grant date fair value minus estimated forfeitures.)