Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016		Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments Net Asset Value		$ 247.8			$ 247.8				$ 322.9
Other investments		$ 148.6			$ 148.6				$ 142.3
Spread on U.S. treasury for discounting deposit liabilities		0.416%			0.416%				0.50%
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value asset - Transfers into Level 3	[1]	$ 1.1			$ 1.2
Fair value asset - Transfers out of Level 3		(0.1)	[1]		(0.1)
Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value asset - Transfers into Level 3					4.1
Fair value asset - Transfers out of Level 3					(3.4)
Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value asset - Transfers into Level 3		3.4		$ 3.0	6.1	[2]	$ 3.0	[2]
Fair value asset - Transfers out of Level 3		$ (6.7)		$ (0.9)	$ (6.7)	[2]	(0.9)
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value asset - Transfers into Level 3							15.5
Fair value asset - Transfers out of Level 3							$ (59.9)

[1] Net transfers include both movement in and movement out of Level 3. Corporate for the three months ended June 30, 2017, includes $1.1 million of incoming transfers partially offset by $0.1 million of outgoing transfers. Other asset-backed securities, for the three months ended June 30, 2017, includes $6.7 million of outgoing transfers partially offset by $3.4 million of incoming transfers. Other asset-backed securities, for the three months ended June 30, 2016, includes $3.0 million of incoming transfers partially offset by $0.9 million of outgoing transfers.

[2] Net transfers include both movement in and movement out of Level 3. Corporate for the six months ended June 30, 2017, includes $1.2 million of incoming transfers partially offset by $0.1 million of outgoing transfers. RMBS for the six months ended June 30, 2017, includes $4.1 million of incoming transfers partially offset by $3.4 million of outgoing transfers. Other asset-backed securities, for the six months ended June 30, 2017, includes $6.7 million of outgoing transfers partially offset by $6.1 million of incoming transfers. Other investments, for the six months ended June 30, 2016, includes $59.9 million of outgoing transfers partially offset by $15.5 million of incoming transfers. Other asset-backed securities, for the six months ended June 30, 2016, includes $3.0 million of incoming transfers partially offset by $0.9 million of outgoing transfers.