Employee Retirement Plans - Target Asset Allocation and Acceptable Ranges (Details)	12 Months Ended
	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	100.00%
Equity
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	40.00%
Target plan asset allocations, range minimum (as a percent)	36.00%
Target plan asset allocations, range maximum (as a percent)	42.00%
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	49.00%
Target plan asset allocations, range minimum (as a percent)	46.00%
Target plan asset allocations, range maximum (as a percent)	52.00%
Limited partnerships
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	5.00%
Target plan asset allocations, range minimum (as a percent)	2.00%
Target plan asset allocations, range maximum (as a percent)	8.00%
Other real assets
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	6.00%
Target plan asset allocations, range minimum (as a percent)	3.00%
Target plan asset allocations, range maximum (as a percent)	9.00%
Cash equivalents and short-term investments
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as a percent)	0.00%
Target plan asset allocations, range minimum (as a percent)	0.00%
Target plan asset allocations, range maximum (as a percent)	5.00%