Income Taxes - Statutory Federal Income Tax Expense, Effective Income Tax Expense, Rates (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
Amount
Statutory Federal Income Tax Expense									$ (107.1)	$ (138.9)	$ (41.8)
Tax-exempt Income and Dividends Received Deduction									$ 4.0	$ 4.3	$ 4.8
Effective Income Tax Rate Reconciliation, Untaxed Earnings on Company-Owned Life Insurance, Percent									(0.50%)	(0.20%)	(0.40%)
Effective Income Tax Rate Reconciliation, Untaxed Earnings on Company-Owned Life Insurance									$ 2.7	$ 1.6	$ 0.8
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent									(0.60%)	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Amount									$ 3.2	$ 0.0	$ 0.0
Stock-Based Compensation									2.2	4.4	1.4
Nondeductible Executive Compensation									(2.7)	(2.5)	(1.4)
Tax Reform									0.0	0.0	26.4
Other, Net									(2.5)	0.6	(0.9)
Income Tax Benefit (Expense)	$ (23.1)	$ (29.8)	$ (33.6)	$ (13.7)	$ (32.2)	$ (32.5)	$ (26.4)	$ (39.4)	$ (100.2)	$ (130.5)	$ (10.7)
Rate
Statutory Federal Income Tax Expense									21.00%	21.00%	21.00%
Tax-exempt Income and Dividends Received Deduction									(0.80%)	(0.70%)	(2.40%)
Stock-Based Compensation									(0.50%)	(0.70%)	(0.70%)
Nondeductible Executive Compensation									0.50%	0.40%	0.70%
Tax Reform									0.00%	0.00%	(13.30%)
Other, Net									0.50%	(0.10%)	0.50%
Effective Income Tax Benefit (Expense) from Continuing Operations									19.60%	19.70%	5.40%