Schedule II - Condensed Financial Information of Registrant Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income (loss)	$ 275.8	$ 202.8	$ 165.5	$ 412.2	$ (106.5)	$ 275.2	$ 197.7	$ 4.0	$ 1,056.4	$ 370.5	$ 560.5
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Income taxes - net									37.2	(69.5)	49.5
Realized Investment Gains Losses From Sales									(36.6)	(58.2)	(211.6)
Accounts payable, accrued expenses and other									88.0	30.0	134.5
Total									936.2	760.5	452.8
Cash flows from investing activities:
Maturities and early calls									779.0	964.0	1,000.6
Net decrease (increase) in short-term investments									(129.7)	314.2	11.8
Total									424.6	239.5	293.9
Cash flows from financing activities:
Issuance of common shares									13.8	13.1	17.8
Redemption of debentures and notes									(8.4)	(4.7)	(3.9)
Origination of Loans to Employee Stock Ownership Plans									0.0	(50.0)	0.0
Dividends on common shares									(538.7)	(498.8)	(198.8)
Other - net									0.2	(6.0)	6.4
Total									(533.1)	(546.5)	(178.5)
Increase (decrease) in cash:									(21.4)	(25.6)	(19.7)
Cash, beginning of year				100.3				125.9	100.3	125.9	145.7
Cash, end of year	78.8				100.3				78.8	100.3	125.9
Parent Company [Member]
Cash flows from operating activities:
Net income (loss)									1,056.4	370.5	560.5
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Accounts receivable									(0.7)	0.2	(0.1)
Income taxes - net									0.4	(23.4)	41.9
Excess of equity in net (income) loss of subsidiaries over cash dividends received									(620.0)	80.0	(201.9)
Realized Investment Gains Losses From Sales									0.2	0.0	0.0
Accounts payable, accrued expenses and other									(1.0)	2.2	7.9
Total									435.3	429.6	408.2
Cash flows from investing activities:
Sale of fixed assets for company use									0.4	0.0	0.0
Net repayment (issuance) of notes to related parties									102.5	(114.2)	7.3
Net decrease (increase) in short-term investments									(10.8)	266.6	(69.0)
Investment In,And Indebtedness Of Related Parties, Net									10.9	(104.0)	(178.7)
Total									(103.0)	(48.3)	(240.4)
Cash flows from financing activities:
Net receipt (repayment) of notes and loans from related parties									(10.3)	50.7	17.3
Issuance of common shares									13.8	13.1	17.8
Redemption of debentures and notes									(6.5)	(4.7)	(3.9)
Origination of Loans to Employee Stock Ownership Plans									0.0	(50.0)	0.0
Dividends on common shares									(538.7)	(498.8)	(198.8)
Other - net									0.2	(3.1)	(0.2)
Total									(541.5)	(492.8)	(167.8)
Increase (decrease) in cash:									(3.1)	(14.9)	0.0
Cash, beginning of year				$ 5.0				$ 20.0	5.0	20.0	19.9
Cash, end of year	$ 1.9				$ 5.0				$ 1.9	$ 5.0	$ 20.0