Investments (Narrative) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) security item	Dec. 31, 2018 USD ($) security item
Investment [Line Items]
Increase (decrease) in gross AFS securities unrealized losses	$ (1,500.0)
Number of partnerships in alternative investment portfolio | item	246	237
Alternative investments as a percentage of overall invested assets	2.00%	1.00%
Fair value of collateral received that we are permitted to sell or re-pledge	$ 548.0
Securities that have been re-pledged	536.0
Investment commitments	2,000.0
Investment commitments for limited partnerships	823.0
Investment commitments for mortgage loans on real estate	669.0
Investment commitments for private placements	$ 528.0
Commercial [Member]
Investment [Line Items]
Number of impaired loans | security	0	0
Residential [Member]
Investment [Line Items]
Number of impaired loans | security	0	0
Impaired financing receivable, allowance	$ 1.0
Maximum [Member] | Residential [Member]
Investment [Line Items]
Impaired financing receivable, allowance		$ 1.0
Federal Home Loan Mortgage Corporation [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	$ 1,400.0	$ 1,400.0
Concentration risk, percentage	1.00%	1.00%
Federal National Mortgage Association [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	$ 1,300.0	$ 1,300.0
Concentration risk, percentage	1.00%	1.00%
Consumer Non-Cyclical Industry [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	$ 15.1	$ 14.5
Concentration risk, percentage	12.00%	13.00%
Financial Service [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	$ 15.3	$ 14.2
Concentration risk, percentage	13.00%	12.00%
Mortgage Loans On Real Estate [Member] | Geographic Concentration Risk [Member] | California [Member] | Commercial [Member]
Investment [Line Items]
Concentration risk, percentage	23.00%	23.00%
Mortgage Loans On Real Estate [Member] | Geographic Concentration Risk [Member] | California [Member] | Residential [Member]
Investment [Line Items]
Concentration risk, percentage	30.00%	34.00%
Mortgage Loans On Real Estate [Member] | Geographic Concentration Risk [Member] | Texas [Member] | Commercial [Member]
Investment [Line Items]
Concentration risk, percentage	12.00%	12.00%
Mortgage Loans On Real Estate [Member] | Geographic Concentration Risk [Member] | Florida [Member] | Residential [Member]
Investment [Line Items]
Concentration risk, percentage	19.00%	19.00%
Corporate Bonds [Member]
Investment [Line Items]
Percentage of fair value rated as investment grade	96.00%	96.00%
Amortized cost of portfolio rated below investment grade	$ 3,300.0	$ 3,200.0
Fair value of portfolio rated below investment grade	$ 3,200.0	$ 3,000.0
MBS [Member]
Investment [Line Items]
Severity of second lien loans	100.00%	100.00%
Severity of first lien loans	30.00%	30.00%