Investments - Mortgage Loans (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Maximum loan to value ratio generally allowed	75.00%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	$ 4,849.1	$ 4,255.6
Collective valuation allowance for losses	(0.9)	(1.1)
Total net commercial mortgage loans	4,848.2	4,254.5
Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4.4	4.6
Total net commercial mortgage loans	4.4	4.6
Non Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4,844.7	4,251.0
Collective valuation allowance for losses	(0.9)	(1.1)
Total net commercial mortgage loans	$ 4,843.8	$ 4,249.9