Fair Value - Separate Accounts Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	$ 4,881.0	$ 4,459.0
Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	4,852.0	4,459.0
Gross transfers into (out of) level 3	1.0	(4.0)
Recurring | Other receivables
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	29.0
Recurring | Cash and cash equivalents
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	188.0	145.0
Recurring | Debt securities
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	4,099.0	3,813.0
Recurring | Equity securities
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	2.0	2.0
Recurring | Common/collective trusts
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	563.0	499.0
Recurring | Level 1
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	1,467.0	1,226.0
Recurring | Level 1 | Cash and cash equivalents
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	2.0	2.0
Recurring | Level 1 | Debt securities
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	1,465.0	1,224.0
Recurring | Level 1 | Equity securities
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0.0	0.0
Recurring | Level 1 | Common/collective trusts
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0.0	0.0
Recurring | Level 2
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	3,385.0	3,233.0
Recurring | Level 2 | Cash and cash equivalents
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	186.0	143.0
Recurring | Level 2 | Debt securities
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	2,634.0	2,589.0
Recurring | Level 2 | Equity securities
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	2.0	2.0
Recurring | Level 2 | Common/collective trusts
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	563.0	499.0
Recurring | Level 3
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0.0	0.0
Gross transfers into (out of) level 3	1.0	(4.0)
Recurring | Level 3 | Cash and cash equivalents
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0.0	0.0
Recurring | Level 3 | Debt securities
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0.0	0.0
Recurring | Level 3 | Equity securities
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0.0	0.0
Recurring | Level 3 | Common/collective trusts
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0.0	0.0
Recurring | Level 3 | Separate Accounts, financial assets
Fair Value, Separate Account Investment [Line Items]
Gross transfers into (out of) level 3	$ 0.0	$ 0.0