Investments - Unrealized Gains and Gross Unrealized Losses in Investment in Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	$ 434.4	$ 486.9
Gross Unrealized Gains	52.9	43.3
Gross Unrealized Losses	(5.6)	(7.0)
Equity Securities at Fair Value	481.7	523.2
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	136.1	179.7
Gross Unrealized Gains	9.6	1.1
Gross Unrealized Losses	(1.3)	(3.7)
Equity Securities at Fair Value	144.4	177.1
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	183.3	181.0
Gross Unrealized Gains	29.2	25.0
Gross Unrealized Losses	(2.9)	(0.5)
Equity Securities at Fair Value	209.6	205.5
Finance, Insurance and Real Estate [Member] | Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	58.1	80.8
Gross Unrealized Gains	2.3	4.9
Gross Unrealized Losses	(0.8)	(0.8)
Equity Securities at Fair Value	59.6	84.9
Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	31.2	18.9
Gross Unrealized Gains	2.3	5.3
Gross Unrealized Losses	0.0	(1.0)
Equity Securities at Fair Value	33.5	23.2
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	18.5	17.1
Gross Unrealized Gains	4.9	2.7
Gross Unrealized Losses	(0.5)	(0.8)
Equity Securities at Fair Value	22.9	19.0
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	7.2
Gross Unrealized Gains	4.6
Gross Unrealized Losses	(0.1)
Equity Securities at Fair Value	$ 11.7
Manufacturing [Member] | Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost		0.7
Gross Unrealized Gains		1.0
Gross Unrealized Losses		0.0
Equity Securities at Fair Value		1.7
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		8.7
Gross Unrealized Gains		3.3
Gross Unrealized Losses		(0.2)
Equity Securities at Fair Value		$ 11.8