Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 24.7us-gaap_ProfitLoss	$ 27.1us-gaap_ProfitLoss
Net unrealized holding gains on investments:
Unrealized holding gains	17.0	23.0
Reclassification adjustments for gains realized in net income	(4.2)	(10.7)
Income tax expense	(4.5)	(4.2)
Total net unrealized holding gains on investments	8.3	8.1
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.4)	(1.4)
Net actuarial loss	2.9	1.6
Income tax expense	(0.5)	0
Total net unrecognized benefit plan obligations	1.0	0.2
Other comprehensive income	9.3	8.3
Comprehensive income	$ 34.0	$ 35.4

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Other comprehensive income reclassification of defined benefit plan net actuarial loss before 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. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount after tax and 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 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 tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.

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Amount of tax (expense) benefit, after reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.

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Amount before tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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Amount before tax of reclassification adjustment from accumulated other comprehensive income (loss) for net transition asset (obligation) related to pension and other postretirement benefit plans.

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Amount before tax and reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.

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The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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