Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

Investments (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
Investments (Narrative) (Details) $ in Millions	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) item
Investment [Line Items]
Increase (decrease) in gross AFS securities unrealized losses			$ 1,500.0
Portion of loss recognized in other comprehensive income		$ 1.0		$ 1.0
Number of partnerships in alternative investment portfolio \| item	239		239		224
Alternative investments as a percentage of overall invested assets			1.00%		1.00%
Fair value of collateral received that we are permitted to sell or re-pledge	$ 528.0		$ 528.0
Securities that have been re-pledged	502.0		502.0
Investment commitments	1,500.0		1,500.0
Investment commitments for limited partnerships	815.0		815.0
Investment commitments for mortgage loans on real estate	379.0		379.0
Investment commitments for private placements	334.0		334.0
Maximum [Member]
Investment [Line Items]
Portion of loss recognized in other comprehensive income	1.0		1.0
Federal Home Loan Mortgage Corporation [Member] \| Invested Assets [Member]
Investment [Line Items]
Fair value	1,400.0		$ 1,400.0		$ 1,300.0
Concentration risk, percentage			1.00%		1.00%
Federal National Mortgage Association [Member] \| Invested Assets [Member]
Investment [Line Items]
Fair value	1,200.0		$ 1,200.0		$ 1,000.0
Concentration risk, percentage			1.00%		1.00%
Consumer Non-Cyclical Industry [Member] \| Invested Assets [Member]
Investment [Line Items]
Fair value	14.5		$ 14.5		$ 15.0
Concentration risk, percentage			13.00%		13.00%
Financial Service [Member] \| Invested Assets [Member]
Investment [Line Items]
Fair value	$ 13.7		$ 13.7
Concentration risk, percentage			12.00%
Utilities Industry [Member] \| Invested Assets [Member]
Investment [Line Items]
Fair value					$ 14.3
Concentration risk, percentage					13.00%
Mortgage Loans On Real Estate [Member] \| Geographic Concentration Risk [Member] \| California [Member]
Investment [Line Items]
Concentration risk, percentage			22.00%		21.00%
Mortgage Loans On Real Estate [Member] \| Geographic Concentration Risk [Member] \| Texas [Member]
Investment [Line Items]
Concentration risk, percentage			12.00%		12.00%
Corporate Bonds [Member]
Investment [Line Items]
Percentage of fair value rated as investment grade	96.00%		96.00%		96.00%
Amortized cost of portfolio rated below investment grade	$ 3,200.0		$ 3,200.0		$ 3,500.0
Fair value of portfolio rated below investment grade	$ 3,100.0		$ 3,100.0		$ 3,500.0
ABS [Member]
Investment [Line Items]
Percentage of fair value rated as investment grade	98.00%		98.00%		98.00%
Amortized cost of portfolio rated below investment grade	$ 46.0		$ 46.0		$ 46.0
Fair value of portfolio rated below investment grade	$ 43.0		$ 43.0		$ 43.0
MBS [Member]
Investment [Line Items]
Projected default rate on existing delinquencies on MBS (low end of range)	10.00%		10.00%		10.00%
Projected default rate on existing delinquencies on MBS (high end of range)	100.00%		100.00%		100.00%
Severity of second lien loans	100.00%		100.00%		100.00%
Severity of first lien loans	30.00%		30.00%		30.00%