Schedule II - Condensed Financial Information of Registrant Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets:
Short-term investments	$ 484.3	$ 354.9
Other assets	748.5	526.3
Total Assets	21,076.3	19,327.1
Liabilities:
Debt	974.0	981.4
Commitments and contingent liabilities
Total Liabilities	15,076.1	14,180.8
Common Shareholders' Equity:
Common stock	303.6	302.7
Additional paid-in capital	1,297.5	1,277.6
Retained earnings	4,386.0	3,849.8
Accumulated other comprehensive income (loss)	77.7	(210.0)
Unallocated ESSOP shares (at cost)	(64.8)	(73.9)
Total Common Shareholders' Equity	6,000.1	5,146.2
Liabilities and Common Shareholders' Equity:
Total Liabilities, Preferred Stock and Common Shareholders' Equity	21,076.3	19,327.1
Parent Company [Member]
Assets:
Bond and notes	10.5	10.5
Short-term investments	17.1	6.2
Cash	1.9	5.0
Investments in, and indebtedness of related parties	7,131.8	6,276.3
Other assets	93.2	86.0
Total Assets	7,254.7	6,384.2
Liabilities:
Accounts payable and accrued expenses	154.1	150.2
Debt	969.6	975.0
Indebtedness to affiliates and subsidiaries	130.8	112.7
Commitments and contingent liabilities
Total Liabilities	1,254.5	1,238.0
Common Shareholders' Equity:
Common stock	303.6	302.7
Additional paid-in capital	1,297.5	1,277.6
Retained earnings	4,386.0	3,849.8
Accumulated other comprehensive income (loss)	77.7	(210.0)
Unallocated ESSOP shares (at cost)	(64.8)	(73.9)
Total Common Shareholders' Equity	6,000.1	5,146.2
Liabilities and Common Shareholders' Equity:
Total Liabilities, Preferred Stock and Common Shareholders' Equity	$ 7,254.7	$ 6,384.2