Investments - Mortgage Loans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 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	$ 4,746.3	$ 4,255.6
Collective valuation allowance for losses	(1.2)	(1.1)
Total net commercial mortgage loans	4,745.1	4,254.5
Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4.5	4.6
Total net commercial mortgage loans	4.5	4.6
Non Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4,741.8	4,251.0
Collective valuation allowance for losses	(1.2)	(1.1)
Total net commercial mortgage loans	$ 4,740.6	$ 4,249.9