Schedule II - Condensed Financial Information of Registrant Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets:
Short-term investments	$ 749.6	$ 484.3
Other assets	790.0	748.5
Total Assets	22,815.2	21,076.3
Liabilities:
Debt	966.4	974.0
Commitments and contingent liabilities
Total Liabilities	16,628.5	15,076.1
Common Shareholders' Equity:
Common stock	304.1	303.6
Additional paid-in capital	1,306.9	1,297.5
Retained earnings	4,394.8	4,386.0
Accumulated other comprehensive income (loss)	284.0	77.7
Unallocated ESSOP shares (at cost)	(103.2)	(64.8)
Total Common Shareholders' Equity	6,186.6	6,000.1
Liabilities and Common Shareholders' Equity:
Total Liabilities, Preferred Stock and Common Shareholders' Equity	22,815.2	21,076.3
Parent Company [Member]
Assets:
Bond and notes	10.5	10.5
Short-term investments	32.7	17.1
Cash	1.9	1.9
Investments in, and indebtedness of related parties	7,668.0	7,131.8
Other assets	113.9	93.2
Total Assets	7,827.1	7,254.7
Liabilities:
Accounts payable and accrued expenses	530.3	154.1
Debt	964.2	969.6
Indebtedness to affiliates and subsidiaries	145.7	130.8
Commitments and contingent liabilities
Total Liabilities	1,640.4	1,254.5
Common Shareholders' Equity:
Common stock	304.1	303.6
Additional paid-in capital	1,306.9	1,297.5
Retained earnings	4,394.8	4,386.0
Accumulated other comprehensive income (loss)	284.0	77.7
Unallocated ESSOP shares (at cost)	(103.2)	(64.8)
Total Common Shareholders' Equity	6,186.6	6,000.1
Liabilities and Common Shareholders' Equity:
Total Liabilities, Preferred Stock and Common Shareholders' Equity	$ 7,827.1	$ 7,254.7