Summary Of Accounting Policies And Accounting Changes - Impact on Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income	$ 1.9	$ 55.6	$ 2.3	$ 43.6	$ 24.3	$ 2.0	$ (3.3)	$ 51.5	$ 103.4	$ 74.5	$ 177.9
Total Comprehensive Income									158.1	175.1	276.5
Accounting Standards Update 2010-26 [Member] | Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income										83.7	184.6
Total Comprehensive Income										184.3	283.2
Accounting Standards Update 2010-26 [Member] | Scenario, Computed Without Change in Accounting Principle [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income									109.7
Total Comprehensive Income									164.4
Accounting Standards Update 2010-26 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income									(6.3)	(9.2)	(6.7)
Total Comprehensive Income									$ (6.3)	$ (9.2)	$ (6.7)