Investments (Amortized Cost and Fair Value of Fixed Maturity Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, cost or amortized cost	$ 243.8
Due after one year through five years, cost or amortized cost	1,709.4
Due after five years through ten years, cost or amortized cost	1,847.2
Due after ten years, cost or amortized cost	3,797.7
Total, cost or amortized cost	7,598.1
Fixed maturity securities, cost or amortized cost	8,888.9	$ 8,870.8
Due in one year or less, fair value	246.4
Due after one year through five years, fair value	1,768.0
Due after five years through ten years, fair value	1,933.4
Due after ten years, fair value	4,460.4
Total, fair value	8,408.2
Total fair value	9,732.4	$ 9,572.1
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	40.1
Fair value	39.5
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	1,114.4
Fair value	1,147.3
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	136.3
Fair value	$ 137.4