Stock-Based Compensation (Schedule of Fair Value of the Restricted Stock Awards Granted During the Periods) (Details) - Restricted Stock [Member] - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Time Vested [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		839,498	461,799
Estimated fair value (in millions)	[1]	$ 16.1	$ 11.4
Time Vested [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		839,498	461,799
Estimated fair value (in millions)	[1]	$ 16.1	$ 11.4
Percentage of shares granted expected to be distributed		95.00%	94.00%
Time Vested [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
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		362,070	152,373
Estimated fair value (in millions)	[1]	$ 7.3	$ 4.0
Performance Shares [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted		362,070	152,373
Estimated fair value (in millions)	[1]	$ 7.3	$ 4.0
Percentage of shares granted expected to be distributed		63.00%	105.00%
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] Represents 100% of the grant date fair value. (We recognize the grant date fair value minus estimated forfeitures.)