SCHEDULE II - Condensed Financial Information of Registrant (Parent Company) - Statement of Cash Flows (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Condensed Financial Statements, Captions [Line Items]
Cash flows from operating activities		$ 495.4	$ 598.3	$ 735.5
Cash flows from investing activities:
Sales of investments		3,253.6	1,823.6	1,480.0
Purchases of investments		(6,482.0)	(6,135.2)	(4,280.7)
Net cash provided by investing activities		(1,781.3)	(1,526.1)	(608.4)
Cash flows from financing activities:
Issuance of notes payable, net		0.0	0.0	145.8
Issuance of common stock		10.4	18.4	16.5
Payments to repurchase common stock		(190.1)	(407.8)	(268.3)
Debt issuance costs		0.0	(1.0)	0.0
Net cash used by financing activities		1,199.1	667.6	210.1
Net increase (decrease) in cash and cash equivalents		(86.8)	(260.2)	337.2
Cash and cash equivalents - unrestricted and held by variable interest entities, beginning of year		731.7	991.9	654.7
Cash and cash equivalents - unrestricted and held by variable interest entities, end of year		644.9	731.7	991.9
Parent Company
Condensed Financial Statements, Captions [Line Items]
Cash flows from operating activities		(143.2)	(153.2)	(136.3)
Cash flows from investing activities:
Sales of investments		24.6	0.0	18.4
Purchases of investments		(11.8)	(50.0)	(18.2)
Net sales of trading securities		7.7	8.4	26.8
Dividends received from consolidated subsidiary	[1]	69.6	328.3	324.7
Net cash provided by investing activities		90.1	286.7	351.7
Cash flows from financing activities:
Issuance of notes payable, net		0.0	0.0	145.8
Issuance of common stock		13.5	21.5	19.0
Payments to repurchase common stock		(190.1)	(407.8)	(268.3)
Common stock dividends paid		(64.8)	(65.7)	(67.0)
Issuance of notes payable to affiliates	[1]	349.5	249.7	308.1
Payments on notes payable to affiliates	[1]	(114.5)	(125.3)	(143.6)
Debt issuance costs		0.0	(1.0)	0.0
Net cash used by financing activities		(6.4)	(328.6)	(6.0)
Net increase (decrease) in cash and cash equivalents		(59.5)	(195.1)	209.4
Cash and cash equivalents - unrestricted and held by variable interest entities, beginning of year		196.2	391.3	181.9
Cash and cash equivalents - unrestricted and held by variable interest entities, end of year		$ 136.7	$ 196.2	$ 391.3

[1] Eliminated in consolidation