Investments (Amortized Cost And Fair Value Of Fixed Maturity Securities In An Unrealized Loss Position By Contractual Maturity) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, cost or amortized cost	$ 22.2
Due after one year through five years, cost or amortized cost	504.0
Due after five years through ten years, cost or amortized cost	412.6
Due after ten years, cost or amortized cost	125.4
Total single maturity date, cost or amortized cost	1,064.2
Total, cost or amortized cost	1,492.5
Due in one year or less, fair value	22.1
Due after one year through five years, fair value	500.2
Due after five years through ten years, fair value	407.6
Due after ten years, fair value	123.7
Total single maturity date, fair value	1,053.6
Total, fair value	1,473.8
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	13.1
Fair value	12.4
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	387.5
Fair value	380.2
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	27.7
Fair value	$ 27.6