Consolidated Statements of Comprehensive Income (unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Change in unrealized gains on securities, tax	$ (60.0)	$ 28.2	$ (129.0)	$ 76.0
Change in unrealized gains on derivative transactions, tax	0.1	0.2	0.3	(5.4)
Change in other-than-temporary impairment gains, tax	(0.2)	0.4	(0.1)	1.3
Change in foreign currency transaction, tax	(0.8)	1.0	(1.4)	1.5
Amortization of pension and postretirement unrecognized net periodic benefit cost and change in funded status, tax	$ (0.1)	$ (0.3)	$ (0.1)	$ (0.3)

Amount of tax expense (benefit), before reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.

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Amount of tax expense (benefit), after reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities. Excludes amounts related to other than temporary impairment (OTTI) loss.

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Amount, after reclassification, of tax expense (benefit) for gain (loss) from derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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Amount, after reclassification adjustment, of tax (expense) benefit for (increase) decrease in accumulated other comprehensive income of defined benefit plan.

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Amount of tax (expense) benefit of other than temporary impairment (OTTI) on investment in available-for-sale debt security, recognized in other comprehensive loss.

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