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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Assets and liabilities measured at fair value on a nonrecurring basis
Fair value of assets measured on nonrecurring basis	$ 0.8	$ 7.7	$ 0.8	$ 7.7
Discount rate (as a percent)				10.00%
Terminal capitalization rate (as a percent)				9.00%
Minimum
Assets and liabilities measured at fair value on a nonrecurring basis
Average market rent growth rate (as a percent)				1.00%
Maximum
Assets and liabilities measured at fair value on a nonrecurring basis
Average market rent growth rate (as a percent)				3.50%
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	$ 0.0	$ 3.0	$ 0.3	$ 3.1