Investments - Unrealized Gains and Gross Unrealized Losses in Investment in Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	$ 476.2	$ 434.4
Gross Unrealized Gains	56.2	52.9
Gross Unrealized Losses	(6.4)	(5.6)
Equity Securities at Fair Value	526.0	481.7
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	207.6	136.1
Gross Unrealized Gains	11.9	9.6
Gross Unrealized Losses	0.0	(1.3)
Equity Securities at Fair Value	219.5	144.4
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	184.9	183.3
Gross Unrealized Gains	23.9	29.2
Gross Unrealized Losses	(5.9)	(2.9)
Equity Securities at Fair Value	202.9	209.6
Finance, Insurance and Real Estate [Member] | Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	51.1	58.1
Gross Unrealized Gains	4.6	2.3
Gross Unrealized Losses	0.0	(0.8)
Equity Securities at Fair Value	55.7	59.6
Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities at Cost	5.5	31.2
Gross Unrealized Gains	1.7	2.3
Gross Unrealized Losses	(0.1)	0.0
Equity Securities at Fair Value	7.1	33.5
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	18.5
Gross Unrealized Gains	6.0	4.9
Gross Unrealized Losses	0.0	(0.5)
Equity Securities at Fair Value	23.1	22.9
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	10.0	7.2
Gross Unrealized Gains	8.1	4.6
Gross Unrealized Losses	(0.4)	(0.1)
Equity Securities at Fair Value	$ 17.7	$ 11.7