Schedule II - Condensed Financial Information of Registrant Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income (loss)	$ 519.7	$ 246.0	$ 397.7	$ (604.8)	$ 275.8	$ 202.8	$ 165.5	$ 412.2	$ 558.6	$ 1,056.4	$ 370.5
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Income taxes - net									(18.3)	37.2	(69.5)
Realized Investment Gains Losses From Sales									(14.2)	(38.6)	(58.2)
Accounts payable, accrued expenses and other									176.2	88.0	30.0
Total									1,185.0	936.2	760.5
Cash flows from investing activities:
Maturities and early calls									1,280.1	779.0	964.0
Net decrease (increase) in short-term investments									(265.0)	(129.7)	314.2
Total									(845.2)	(424.6)	(239.5)
Cash flows from financing activities:
Issuance of common shares									6.7	13.8	13.1
Redemption of debentures and notes									(8.6)	(8.4)	(4.7)
Origination of Loans to Employee Stock Ownership Plans									(50.0)	0.0	(50.0)
Dividends on common shares									(250.1)	(538.7)	(498.8)
Other - net									2.0	0.2	(6.0)
Total									(300.0)	(533.1)	(546.5)
Increase (decrease) in cash:									39.8	(21.4)	(25.6)
Cash, beginning of year				78.8				100.3	78.8	100.3	125.9
Cash, end of year	118.7				78.8				118.7	78.8	100.3
Parent Company [Member]
Cash flows from operating activities:
Net income (loss)									558.6	1,056.4	370.5
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Accounts receivable									0.3	(0.7)	0.2
Income taxes - net									4.4	0.4	(23.4)
Excess of equity in net (income) loss of subsidiaries over cash dividends received									(62.3)	(620.0)	80.0
Realized Investment Gains Losses From Sales									0.0	0.2	0.0
Accounts payable, accrued expenses and other									4.6	(1.0)	2.2
Total									505.7	435.3	429.6
Cash flows from investing activities:
Sale of fixed assets for company use									0.0	0.4	0.0
Purchase of fixed assets for company use									(3.2)	0.0	0.0
Net repayment (issuance) of notes to related parties									(275.0)	102.5	(114.2)
Net decrease (increase) in short-term investments									(15.6)	(10.8)	266.6
Investment In,And Indebtedness Of Related Parties, Net									37.7	10.9	(104.0)
Total									(256.1)	103.0	48.3
Cash flows from financing activities:
Net receipt (repayment) of notes and loans from related parties									48.4	(10.3)	50.7
Issuance of common shares									6.7	13.8	13.1
Redemption of debentures and notes									(6.5)	(6.5)	(4.7)
Origination of Loans to Employee Stock Ownership Plans									(50.0)	0.0	(50.0)
Dividends on common shares									(250.1)	(538.7)	(498.8)
Other - net									1.8	0.2	(3.1)
Total									(249.6)	(541.5)	(492.8)
Increase (decrease) in cash:									0.0	(3.1)	(14.9)
Cash, beginning of year				$ 1.9				$ 5.0	1.9	5.0	20.0
Cash, end of year	$ 1.9				$ 1.9				$ 1.9	$ 1.9	$ 5.0