Investments - Unrealized Gains and Gross Unrealized Losses in Investment in Equity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Equity Securities at Cost	$ 0.0	$ 476.2
Equity Securities at Fair Value	$ 684.4	526.0
Exchange Traded Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Equity Securities at Cost		207.6
Equity Securities at Fair Value		219.5
Limited Liability Companies and Limited Partnerships [Member]
Debt Securities, Available-for-sale [Line Items]
Equity Securities at Cost		184.9
Equity Securities at Fair Value		202.9
Finance, Insurance and Real Estate [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Equity Securities at Cost		51.1
Equity Securities at Fair Value		55.7
Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Equity Securities at Cost		5.5
Equity Securities at Fair Value		7.1
All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Equity Securities at Cost		17.1
Equity Securities at Fair Value		23.1
All Industries Other than Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Equity Securities at Cost		10.0
Equity Securities at Fair Value		$ 17.7