Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Nonrecurring Basis (Details) - Nonrecurring Fair Value Measurements - Fair value hierarchy Level 3 - Mortgage loans - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Assets and liabilities measured at fair value on a nonrecurring basis
Fair value of assets measured on nonrecurring basis	$ 6.1	$ 0.8	$ 6.1	$ 0.8
Net realized capital gains (losses)
Assets and liabilities measured at fair value on a nonrecurring basis
Net (gain) loss due to change in fair value of assets measured on nonrecurring basis	$ 0.1	$ 0.0	$ 0.7	$ 0.3