Retirement Plans - Weighted Average Asset Allocation and Target Asset Allocation by Asset Category (Details)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Summary of weighted average asset allocation and target asset allocations by asset category
Asset Allocations	100.00%		100.00%
Target Asset Allocations	100.00%		100.00%
Equity securities and diversified growth funds
Summary of weighted average asset allocation and target asset allocations by asset category
Asset Allocations	64.10%	[1]	60.90%	[1]
Target Asset Allocations	65.00%	[1]	60.00%	[1]
Debt securities
Summary of weighted average asset allocation and target asset allocations by asset category
Asset Allocations	35.10%	[2]	38.30%	[2]
Target Asset Allocations	35.00%	[2]	40.00%	[2]
Other
Summary of weighted average asset allocation and target asset allocations by asset category
Asset Allocations	0.80%		0.80%
Target Asset Allocations	0.00%		0.00%

[1] Diversified growth funds refer to actively managed absolute return funds that hold a combination of equity and debt securities

[2] Includes investments in funds with the objective to provide leveraged returns to U.K. government fixed income securities and U.K. government indexed-linked securities.