Retirement Benefits - Additional Information (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Percentage of excess gains and losses over benefit obligation or market value of assets amortized		10.00%
Costs of defined contribution plans		$ 10,200	$ 10,100	$ 10,300
Number of participants covering under multi employer pension plan | employee		355
Costs of bargaining unit-sponsored multi-employer plans and defined contribution plans		$ 1,000	1,300	1,000
Weighted average annual rate of increase in the per capita cost of covered health care benefits assumed		6.21%
Assumed decrease of weighted average health care cost trend rate		4.50%
Year that reaches assumed decrease of weighted average health care cost trend rate		2038
Increase in service and interest cost components of the net periodic benefit cost, as 1% increase in health care cost trend rates		$ 200
Increase in accumulated postretirement benefit obligation, as 1% increase in health care cost trend rates		2,300
Decrease in service and interest cost components of the net periodic benefit cost, as 1% decrease in health care cost trend rates		100
Decrease in accumulated postretirement benefit obligation, as 1% decrease in health care cost trend rates		2,000
Future estimated benefit payments for year 2018		13,600
Future estimated benefit payments for year 2019		11,000
Future estimated benefit payments for year 2020		11,300
Future estimated benefit payments for year 2021		11,000
Future estimated benefit payments for year 2022		11,000
Future estimated benefit payments for years 2022 to 2026		54,700
Defined Contribution Plan
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, expected contribution for next fiscal year		11,000
401k
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, expected contribution for next fiscal year		$ 8,500
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocation of fund assets in equities		44.00%
Equity Securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target allocation of fund assets in equities		40.00%
Equity Securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target allocation of fund assets in equities		60.00%
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocation of fund assets in equities		56.00%
Fixed Income Securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target allocation of fund assets in equities		40.00%
Fixed Income Securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target allocation of fund assets in equities		60.00%
Pension Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for all defined benefit pension plans	$ 176,700	$ 182,700	176,700
Defined benefit plan, expected contribution for next fiscal year		5,500
Pension Benefit Plan | U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Lump sum payment		0	11,126
Settlement loss recognized	$ 3,500	0	3,339	$ 0
Other Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Lump sum payment		0	$ 0
Defined benefit plan, expected contribution for next fiscal year		$ 100