Retirement benefits Weighted Average Asset Allocation of Defined Benefit Retirement Plans and Weighted Average Assumptions(Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	100.00%	100.00%
Asset category, Target
Target (as a percent)	100.00%
Benefit obligation
Discount rate (as a percent)	4.22%	5.09%	4.13%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Net periodic benefit cost (years ended)
Discount rate (as a percent)	5.09%	4.13%	5.19%
Expected return on plan assets (as a percent)	7.75%	7.75%	7.75%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Pension benefits | Equity securities managers
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	73.00%	73.00%
Asset category, Target
Target (as a percent)	70.00%
Target minimum range (as a percent)	65.00%
Target maximum range (as a percent)	75.00%
Pension benefits | Fixed income securities managers
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	27.00%	27.00%
Asset category, Target
Target (as a percent)	30.00%
Target minimum range (as a percent)	25.00%
Target maximum range (as a percent)	35.00%
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	100.00%	100.00%
Asset category, Target
Target (as a percent)	100.00%
Benefit obligation
Discount rate (as a percent)	4.17%	5.03%	4.07%
Net periodic benefit cost (years ended)
Discount rate (as a percent)	5.03%	4.07%	4.90%
Expected return on plan assets (as a percent)	7.75%	7.75%	7.75%
Other benefits | Equity securities managers
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	72.00%	74.00%
Asset category, Target
Target (as a percent)	70.00%
Target minimum range (as a percent)	65.00%
Target maximum range (as a percent)	75.00%
Other benefits | Fixed income securities managers
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	28.00%	26.00%
Asset category, Target
Target (as a percent)	30.00%
Target minimum range (as a percent)	25.00%
Target maximum range (as a percent)	35.00%