Other Retirement Plans (Schedule of Multiemployer Pension Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Multiemployer Plans [Line Items]
Multiemployer pension plans, contributions (in dollars)	$ 37.5	$ 72.6	$ 99.7
Multiemployer plan, pension, insignificant, employer contribution, cost	$ 5.0	$ 10.7	13.5
Central Pension Fund Of IUOE And Participating Employers [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	366052390
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Jan. 31, 2020	Jan. 31, 2019
Multiemployer plan, pension, significant, employer contribution, cost	$ 5.6	$ 12.6	20.4
IBEW Local 1249 Pension Plan [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	156035161
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 02, 2021
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 3.7	$ 3.2	2.2
Pipeline Industry Pension Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	736146433
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 2.6	$ 9.6	20.7
Multiemployer pension plans, Company contributions greater than 5% of total plan contributions	true	true
Teamsters National Pipe Line Pension Plan [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	461102851
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 1.8	$ 4.5	7.4
Multiemployer pension plans, Company contributions greater than 5% of total plan contributions	true	true
Laborers' District Council of Western Pennsylvania Pension Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	256135576
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	Implemented
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Yellow	Yellow
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 1.8	$ 1.9	1.4
National Electrical Benefit Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	530181657
Multiemployer pension plans, plan number	001
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 1.6	$ 1.2	1.4
National Electrical Benefit Fund [Member] | Maximum [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, collective bargaining arrangement, expiration date	Dec. 31, 2023
West Virginia Laborers' Pension Trust Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	556026775
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Mar. 31, 2020	Mar. 31, 2019
Multiemployer plan, pension, significant, employer contribution, cost	$ 1.4	$ 4.9	4.5
Multiemployer pension plans, Company contributions greater than 5% of total plan contributions	true	true
International Union of Operating Engineers Local 132 Pension Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	556015364
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Mar. 31, 2020	Mar. 31, 2019
Multiemployer plan, pension, significant, employer contribution, cost	$ 1.2	$ 5.0	5.6
Multiemployer pension plans, Company contributions greater than 5% of total plan contributions	true	true
Midwest Operating Engineers Pension Trust Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	366140097
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer plans, pension protection act zone status, extended amortization provisions	true	true
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Yellow
Multiemployer plan, pension, significant, certified zone status, date	Mar. 31, 2020	Mar. 31, 2019
Multiemployer plan, pension, significant, employer contribution, cost	$ 1.1	$ 1.8	1.5
Laborers' Local Union No. 158 Pension Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	236580323
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 0.9	$ 1.5	1.5
Multiemployer pension plans, Company contributions greater than 5% of total plan contributions	true	true
Central Laborers Pension Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	376052379
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer plans, pension protection act zone status, extended amortization provisions	true	true
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	Implemented
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Yellow	Yellow
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 0.8	$ 1.3	0.9
Laborers' National Pension Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	751280827
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	Implemented
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Red	Red
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 0.8	$ 3.0	4.1
Multiemployer pension plans, Company contributions greater than 5% of total plan contributions		true
Employer- Teamsters Local Nos. 175 & 505 Pension Trust Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	556021850
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer plans, pension protection act zone status, extended amortization provisions	true	true
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	Implemented
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Red	Red
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 0.6	$ 1.7	1.5
Multiemployer pension plans, Company contributions greater than 5% of total plan contributions	true	true
Laborers District Council & Contractors Pension Fund of Ohio [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	316129964
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Dec. 31, 2019	Dec. 31, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 0.4	$ 1.4	1.8
Michigan Laborers Pension Fund [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	386233976
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer plans, pension protection act zone status, extended amortization provisions		true
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	Implemented
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Yellow	Yellow
Multiemployer plan, pension, significant, certified zone status, date	Aug. 31, 2020	Aug. 31, 2019
Multiemployer plan, pension, significant, employer contribution, cost	$ 0.4	$ 1.1	1.5
Laborers' Pension Fund of Roanoke, Virginia [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, employer identification number	546111015
Multiemployer pension plans, plan number	001
Multiemployer pension plans, collective bargaining arrangement, expiration date	May 31, 2023
Multiemployer pension plans, funding improvement plan and rehabilitation plan status	NA
Multiemployer pension plans, surcharge	No
Multiemployer plan, pension, significant, certified zone status [Fixed List]	Green	Green
Multiemployer plan, pension, significant, certified zone status, date	Sep. 30, 2019	Sep. 30, 2018
Multiemployer plan, pension, significant, employer contribution, cost	$ 0.3	$ 1.5	2.5
Multiemployer pension plans, Company contributions greater than 5% of total plan contributions	true	true
Pension Plans [Member]
Multiemployer Plans [Line Items]
Multiemployer pension plans, contributions (in dollars)	$ 30.0	$ 66.9	$ 92.4