Investments - Net Realized Gains (Losses), Including Other-Than-Temporary Impairments (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net realized gains (losses) related to sales and other:
Net realized gains (losses) on investments	$ 18,100,000	$ 15,100,000	$ (40,000,000.0)	$ 62,600,000
Net realized losses related to impairments:
Net realized losses related to impairments	(1,500,000)	(200,000)	(14,600,000)	(1,100,000)
Total net realized gains (losses)	16,600,000	14,900,000	(54,600,000)	61,500,000
Gains on equity investment holdings accounted for under alternative measurement	0	0	2,200,000	10,400,000
Credit impairment charges recognized in net realized gains (losses)	1,500,000		1,500,000
Fixed maturity securities
Net realized gains (losses) related to sales and other:
Net realized gains (losses) on investments	1,800,000	7,400,000	7,600,000	15,100,000
Net realized losses related to impairments:
Net realized losses related to impairments	(1,500,000)	(200,000)	(3,200,000)	(1,100,000)
Equity securities
Net realized gains (losses) related to sales and other:
Net realized gains (losses) on investments	16,900,000	9,400,000	(9,900,000)	46,300,000
Commercial mortgage loans on real estate
Net realized gains (losses) related to sales and other:
Net realized gains (losses) on investments	(2,800,000)	0	(4,600,000)	0
Other investments
Net realized gains (losses) related to sales and other:
Net realized gains (losses) on investments	2,200,000	300,000	(800,000)	5,400,000
Net realized losses related to impairments:
Other investments	0	0	(11,400,000)	0
Consolidated investment entities
Net realized gains (losses) related to sales and other:
Net realized gains (losses) on investments	$ 0	$ (2,000,000.0)	$ (32,300,000)	$ (4,200,000)