Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (604.8)	$ 412.2
Unrealized gains (losses) on securities:
Unrealized gains (losses) on securities before reclassifications	(173.2)	174.4
Amounts reclassified as realized investment gains from sales in the statements of income	(0.3)	0.8
Pretax unrealized gains (losses) on securities	(173.5)	175.3
Less: Deferred income taxes (credits)	(36.3)	36.9
Net unrealized gains (losses) on securities, net of tax	(137.1)	138.3
Defined benefit pension plans:
Net pension adjustment before reclassifications	0.0	(2.6)
Amounts reclassified as underwriting, acquisition, and other expenses in the statements of income	0.9	1.0
Pretax net adjustment related to defined benefit pension plans	0.9	(1.5)
Deferred income taxes (credits)	0.1	(0.3)
Net adjustment related to defined benefit pension plans, net of tax	0.7	(1.2)
Foreign currency translation and other adjustments	(10.5)	2.3
Total other comprehensive income (loss)	(146.9)	139.3
Comprehensive Income (Loss)	$ (751.8)	$ 551.6