Employee Benefits - Weighted-average assumptions used to determine future benefit obligations (Details)	Dec. 31, 2014	Dec. 31, 2013
U.K. [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.70%	4.55%
Underlying price inflation	1.95%	2.40%
U.K. [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.35%	3.70%
U.K. [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	4.05%	4.40%
U.S. [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.37%	3.97%
U.S. [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	4.08%	4.87%
Other [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.03%	3.60%
Rate of compensation increase	2.25%	2.25%
Underlying price inflation	2.00%	1.50%
Other [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.91%	4.71%
Rate of compensation increase	3.50%	3.50%
Underlying price inflation	2.50%	2.50%