Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 99.3	$ 78.4	$ 167.0	$ 123.6
Available-for-sale securities:
Net (depreciation) appreciation during the period	(49.0)	19.4	(172.9)	38.3
Change in other-than-temporary impairment losses recognized in other comprehensive income (loss)	(0.1)	0.4	0.3	0.5
Total available-for-sale securities	(49.1)	19.8	(172.6)	38.8
Pension and postretirement benefits:
Net change in net actuarial loss and prior service cost	0.9	2.4	(1.4)	4.7
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period	(0.5)	(4.5)	(1.0)	0.6
Total other comprehensive (loss) income, net of tax	(48.7)	17.7	(175.0)	44.1
Comprehensive income (loss)	$ 50.6	$ 96.1	$ (8.0)	$ 167.7

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Other comprehensive income loss amortization adjustment from AOCI pension and other postretirement benefit plans for net actuarial gain (loss) and net prior service cost credit net of tax.

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Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities and unrealized holding gain (loss) related to transfers of securities into the available-for-sale classification and out of the held-to-maturity classification.

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Amount after tax and 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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