Investments - Unrealized Gains and Gross Unrealized Losses in Investment in Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	$ 486.9	$ 561.5
Gross Unrealized Gains	43.3	75.7
Gross Unrealized Losses	(7.0)	(5.0)
Equity Securities at Fair Value	523.2	632.2
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	179.7	195.2
Gross Unrealized Gains	1.1	8.2
Gross Unrealized Losses	(3.7)	(0.7)
Equity Securities at Fair Value	177.1	202.7
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	181.0	160.3
Gross Unrealized Gains	25.0	27.7
Gross Unrealized Losses	(0.5)	(1.8)
Equity Securities at Fair Value	205.5	186.2
Finance, Insurance and Real Estate [Member] | Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	80.8	85.5
Gross Unrealized Gains	4.9	5.2
Gross Unrealized Losses	(0.8)	(1.0)
Equity Securities at Fair Value	84.9	89.7
Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	18.9	34.3
Gross Unrealized Gains	5.3	6.2
Gross Unrealized Losses	(1.0)	0.0
Equity Securities at Fair Value	23.2	40.5
All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	17.1	16.3
Gross Unrealized Gains	2.7	3.5
Gross Unrealized Losses	(0.8)	0.0
Equity Securities at Fair Value	19.0	19.8
All Industries Other than Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	9.4
Gross Unrealized Gains	4.3
Gross Unrealized Losses	(0.2)
Equity Securities at Fair Value	$ 13.5
Manufacturing [Member] | Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost		43.4
Gross Unrealized Gains		14.9
Gross Unrealized Losses		(1.1)
Equity Securities at Fair Value		57.2
All Industries Other than Manufacturing and Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost		26.5
Gross Unrealized Gains		10.0
Gross Unrealized Losses		(0.4)
Equity Securities at Fair Value		$ 36.1