Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 412.2	$ 4.0
Unrealized gains (losses) on securities:
Unrealized gains (losses) on securities before reclassifications	174.4	(148.6)
Amounts reclassified as realized investment gains from sales in the statements of income	0.8	(0.5)
Pretax unrealized gains (losses) on securities	175.3	(149.2)
Deferred income taxes (credits)	36.9	(31.3)
Net unrealized gains (losses) on securities, net of tax	138.3	(117.8)
Defined benefit pension plans:
Net pension adjustment before reclassifications	(2.6)	0.0
Amounts reclassified as underwriting, acquisition, and other expenses in the statements of income	1.0	0.8
Net adjustment related to defined benefit pension plans	(1.5)	0.8
Deferred income taxes (credits)	(0.3)	0.1
Net adjustment related to defined benefit pension plans, net of tax	(1.2)	0.6
Foreign currency translation and other adjustments	2.3	(3.7)
Net adjustments	139.3	(120.9)
Comprehensive Income (Loss)	$ 551.6	$ (116.8)