Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 397.7	$ 165.5	$ (207.0)	$ 577.7
Unrealized gains (losses) on securities:
Unrealized gains (losses) on securities before reclassifications	462.4	138.8	289.2	313.2
Amounts reclassified as realized investment gains from sales in the statements of income	8.8	2.4	8.5	3.3
Pretax unrealized gains (losses) on securities	471.3	141.2	297.7	316.5
Less: Deferred income taxes (credits)	99.2	29.7	62.8	66.7
Net unrealized gains (losses) on securities, net of tax	372.0	111.4	234.8	249.8
Defined benefit pension plans:
Net pension adjustment before reclassifications	0.0	0.0	0.0	(2.6)
Amounts reclassified as underwriting, acquisition, and other expenses in the statements of income	0.9	1.0	1.8	2.0
Pretax net adjustment related to defined benefit pension plans	0.9	1.0	1.8	(0.5)
Deferred income taxes (credits)	0.1	0.2	0.3	(0.1)
Net adjustment related to defined benefit pension plans, net of tax	0.7	0.8	1.4	(0.4)
Foreign currency translation and other adjustments	4.0	3.1	(6.5)	5.4
Total other comprehensive income (loss)	376.8	115.4	229.8	254.8
Comprehensive Income (Loss)	$ 774.5	$ 280.9	$ 22.7	$ 832.6