Shareholders' Equity (Summary Of Preferred Stock) (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Common Stock Held in Trust	$ 14.0	$ 14.1
Preferred stock, authorized shares	4,400	4,400
Carrying value, Preferred stock	$ 566.0	$ 710.0
Series A Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Rate	> of 4.0% or 3mL+0.52%
Earliest redemption date	Dec. 15, 2011
Preferred stock, authorized shares	140,000
Outstanding Shares at December 31, 2012	66,139
Carrying value, Preferred stock	$ 67.0	67.0
Series F Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Earliest redemption date	Jun. 15, 2017
Preferred stock, authorized shares	0
Outstanding Shares at December 31, 2012	0
Carrying value, Preferred stock	$ 0.0	144.0
Preferred Stock, Dividend Rate, Percentage	7.90%
Series G Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Earliest redemption date	Mar. 15, 2023
Preferred stock, authorized shares	200,000
Outstanding Shares at December 31, 2012	138,391
Carrying value, Preferred stock	$ 138.0	138.0
Preferred Stock, Dividend Rate, Percentage	6.30%
Series H Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Earliest redemption date	Jun. 15, 2019
Preferred stock, authorized shares	126,221
Outstanding Shares at December 31, 2012	126,221
Carrying value, Preferred stock	$ 126.0	126.0
Preferred Stock, Dividend Rate, Percentage	5.75%
Series I Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Earliest redemption date	Jun. 15, 2023
Preferred stock, authorized shares	300,893
Outstanding Shares at December 31, 2012	98,555
Carrying value, Preferred stock	$ 99.0	99.0
Preferred Stock, Dividend Rate, Percentage	5.80%
Series J Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Earliest redemption date	Sep. 15, 2023
Preferred stock, authorized shares	195,152
Outstanding Shares at December 31, 2012	136,368
Carrying value, Preferred stock	$ 136.0	$ 136.0
Preferred Stock, Dividend Rate, Percentage	7.20%