Derivative Financial Instruments - Credit Related Derivative Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Index credit default swaps
Derivative [Line Items]
Derivative notional amount	$ 129.0	$ 146.0
Single name credit default swaps
Derivative [Line Items]
Derivative notional amount	218.2	383.6
Investment Grade | Index credit default swaps
Derivative [Line Items]
Derivative notional amount	3.0	54.0
Investment Grade | Single name credit default swaps
Derivative [Line Items]
Derivative notional amount	129.1	122.4
Non-investment Grade | Index credit default swaps
Derivative [Line Items]
Derivative notional amount	126.0	92.0
Non-investment Grade | Single name credit default swaps
Derivative [Line Items]
Derivative notional amount	$ 89.1	$ 261.2