Schedule 2 - Parent Company Financial Statements - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Investments in Subsidiaries	$ 187.0	$ 161.0
Fixed Maturities at Fair Value (Amortized Cost: 2018 – $12.6; 2017 – $0.3)	6,424.2	5,382.7
Fair Value Option Investments	0.0	77.5
Short-term Investments	286.1	235.5
Cash	75.1	45.7	$ 115.7	$ 161.7
Other Receivables	245.4	194.3
Current Income Tax Assets	38.9	6.1
Other Assets	526.5	270.9
Total Assets	11,544.9	8,376.2
LIABILITIES AND SHAREHOLDERS’ EQUITY
Deferred Income Tax Liabilities	26.2	14.8
Accrued Expenses and Other Liabilities	697.3	453.7
Total Liabilities	8,494.8	6,260.6
Shareholders’ Equity:
Additional Paid-in Capital	1,666.3	673.1
Retained Earnings	1,355.5	1,243.0
Accumulated Other Comprehensive Income	21.8	194.4
Stockholders' Equity Attributable to Parent	3,050.1	2,115.6
Total Liabilities and Shareholders’ Equity	11,544.9	8,376.2
Parent Company [Member]
ASSETS
Investments in Subsidiaries	3,736.0	2,646.0
Fixed Maturities at Fair Value (Amortized Cost: 2018 – $12.6; 2017 – $0.3)	12.6	0.3
Equity Securities at Fair Value	20.4	6.7
Fair Value Option Investments	0.0	77.5
Short-term Investments	64.6	104.4
Cash	2.9	8.4	15.7	$ 15.9
Other Receivables	5.7	3.8
Other Assets	14.4	5.8
Total Assets	3,856.6	2,852.9
LIABILITIES AND SHAREHOLDERS’ EQUITY
Current Income Tax Liability	40.5	14.0
Deferred Income Tax Liabilities	20.2	16.9
Liabilities for Benefit Plans	98.2	104.9
Accrued Expenses and Other Liabilities	20.1	9.2
Total Liabilities	806.5	737.3
Shareholders’ Equity:
Common Stock	6.5	5.1
Additional Paid-in Capital	1,666.3	673.1
Retained Earnings	1,355.5	1,243.0
Accumulated Other Comprehensive Income	21.8	194.4
Stockholders' Equity Attributable to Parent	3,050.1	2,115.6
Total Liabilities and Shareholders’ Equity	3,856.6	2,852.9
Parent Company [Member] | Term Loan Due 2020 [Member]
LIABILITIES AND SHAREHOLDERS’ EQUITY
Notes Payable		34.9	$ 0.0
Parent Company [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]
LIABILITIES AND SHAREHOLDERS’ EQUITY
Notes Payable	448.4	448.1
Parent Company [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
LIABILITIES AND SHAREHOLDERS’ EQUITY
Notes Payable	$ 144.2	$ 144.2