Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net amount of assets (liabilities) of cash, investment income receivable and amounts due to/from brokers, excluded from fair values of pension benefit assets and other benefit assets	$ 2.8	$ 3.4
Contributions made to 401k retirement benefit plans	$ 131.5	125.4	$ 111.1
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average target allocation for plan assets	45.00%
Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average target allocation for plan assets	46.00%
All Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted-average target allocation for plan assets	9.00%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated net actuarial loss that will be reclassified from accumulated other comprehensive income into net periodic benefit costs over next year	$ 18.0
Estimated prior service credit that will be reclassified from accumulated other comprehensive income into net periodic benefit costs over next year	0.6
Accumulated benefit obligation for the defined benefit pension plans	1,821.1	1,829.6
Settlement loss	7.3	6.5	5.2
Tax deductible discretionary contributions	11.1	$ 3.7	$ 3.6
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation	100.6
Accumulated benefit obligation	98.5
Fair value of plan assets	36.8
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated net actuarial loss that will be reclassified from accumulated other comprehensive income into net periodic benefit costs over next year	11.5
Estimated prior service credit that will be reclassified from accumulated other comprehensive income into net periodic benefit costs over next year	13.6
Impact on postretirement benefit obligation of one percentage point increase in cost trend	42.2
Impact on service and interest costs of one percentage point increase in cost trend	1.8
Impact on postretirement benefit obligation of one percentage point decrease in cost trend	36.2
Impact on service and interest costs of one percentage point decrease in cost trend	$ 1.5
Other Benefits [Member] | Pre-Medicare [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate to be used for next year to measure expected cost of other benefits	8.00%
Ultimate health care cost trend rate	4.50%
Year in which ultimate trend rate reached	2028
Other Benefits [Member] | Post-Medicare [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate to be used for next year to measure expected cost of other benefits	6.00%
Ultimate health care cost trend rate	4.50%
Year in which ultimate trend rate reached	2024