Investments - Unrealized Gains and Gross Unrealized Losses in Investment in Equity Securities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	$ 462.7	$ 367.3
Equity Securities at Fair Value	521.9	397.3
Preferred Stocks Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	75.4	94.4
Gross Unrealized Gains	3.9	1.0
Gross Unrealized Losses	(0.1)	(8.7)
Equity Securities at Fair Value	79.2	86.7
Preferred Stocks Other Industries [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	18.4	18.0
Gross Unrealized Gains	3.8	2.6
Gross Unrealized Losses	(0.9)	(0.1)
Equity Securities at Fair Value	21.3	20.5
Common Stocks Manufacturing [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	67.0	64.6
Gross Unrealized Gains	20.9	18.9
Gross Unrealized Losses	(0.4)	(0.1)
Equity Securities at Fair Value	87.5	83.4
Common Stock Other Industries [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	59.1	41.4
Gross Unrealized Gains	8.1	7.4
Gross Unrealized Losses	(0.5)	(1.8)
Equity Securities at Fair Value	66.7	47.0
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	119.6	66.0
Gross Unrealized Gains	6.3	0.6
Gross Unrealized Losses	0	0
Equity Securities at Fair Value	125.9	66.6
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	123.2	82.9
Gross Unrealized Gains	19.5	11.7
Gross Unrealized Losses	(1.4)	(1.5)
Equity Securities at Fair Value	141.3	93.1
Investments in Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	462.7	367.3
Gross Unrealized Gains	62.5	42.2
Gross Unrealized Losses	(3.3)	(12.2)
Equity Securities at Fair Value	$ 521.9	$ 397.3