Derivative Financial Instruments (Credit Related Derivative Contracts) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Index credit default swaps
Derivative [Line Items]
Notional amount	$ 18.0	$ 129.0
Single name credit default swaps
Derivative [Line Items]
Notional amount	75.3	218.2
Investment Grade | Index credit default swaps
Derivative [Line Items]
Notional amount	3.0	3.0
Investment Grade | Single name credit default swaps
Derivative [Line Items]
Notional amount	32.5	129.1
Non-investment grade | Index credit default swaps
Derivative [Line Items]
Notional amount	15.0	126.0
Non-investment grade | Single name credit default swaps
Derivative [Line Items]
Notional amount	39.9	89.1
Unrated | Index credit default swaps
Derivative [Line Items]
Notional amount	0.0	0.0
Unrated | Single name credit default swaps
Derivative [Line Items]
Notional amount	$ 2.9	$ 0.0