Multiemployer plans (Details 13) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 56,745,000	$ 44,527,000	$ 39,112,000
Alaska Laborers-Employers Retirement Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	916028298
Multiemployer plan number	001
Multiemployer plans, certified zone status	Yellow	Yellow
Multiemployer plans, certified zone status, date	Jun. 30, 2017	Jun. 30, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	$ 690,000	$ 766,000	917,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Dec. 31, 2018
Edison Pension Plan [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	936061681
Multiemployer plan number	001
Multiemployer plans, certified zone status	Green	Green
Multiemployer plans, certified zone status, date	Dec. 31, 2017	Dec. 31, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	No
Multiemployer plan, period contributions	$ 12,725,000	$ 6,242,000	5,517,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Jun. 30, 2019
IBEW Local No. 82 Pension Plan [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	316127268
Multiemployer plan number	001
Multiemployer plans, certified zone status	Green	Green
Multiemployer plans, certified zone status, date	Jun. 30, 2017	Jun. 30, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	No
Multiemployer plan, period contributions	$ 1,757,000	$ 2,560,000	2,252,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Dec. 01, 2019
Local Union 212 IBEW Pension Trust Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	316127280
Multiemployer plan number	001
Multiemployer plans, certified zone status	Green	Yellow
Multiemployer plans, certified zone status, date	Apr. 30, 2016	Apr. 30, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan	No
Multiemployer plan, period contributions	$ 1,312,000	$ 1,146,000	937,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Jun. 02, 2019
IBEW Local No. 357 Pension Plan A [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	886023284
Multiemployer plan number	001
Multiemployer plans, certified zone status	Green	Green
Multiemployer plans, certified zone status, date	Dec. 31, 2017	Dec. 31, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	No
Multiemployer plan, period contributions	$ 3,286,000	$ 3,016,000	1,896,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	May 31, 2018
IBEW Local 648 Pension Plan [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	316134845
Multiemployer plan number	001
Multiemployer plans, certified zone status	Red	Red
Multiemployer plans, certified zone status, date	Feb. 28, 2017	Feb. 29, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	$ 2,254,000	$ 773,000	745,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Sep. 02, 2018
Idaho Plumbers and Pipefitters Pension Plan [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	826010346
Multiemployer plan number	001
Multiemployer plans, certified zone status	Green	Green
Multiemployer plans, certified zone status, date	May 31, 2017	May 31, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	No
Multiemployer plan, period contributions	$ 1,156,000	$ 1,221,000	1,169,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Sep. 30, 2019
Minnesota Teamsters Construction Division Pension Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	416187751
Multiemployer plan number	001
Multiemployer plans, certified zone status	Green	Green
Multiemployer plans, certified zone status, date	Nov. 30, 2016	Nov. 30, 2015
Multiemployer plans, funding improvement plan and rehabilitation plan	No
Multiemployer plan, period contributions	$ 826,000	$ 690,000	737,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Apr. 30, 2019
National Automatic Sprinkler Industry Pension Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	526054620
Multiemployer plan number	001
Multiemployer plans, certified zone status	Red	Red
Multiemployer plans, certified zone status, date	Dec. 31, 2017	Dec. 31, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	$ 718,000	$ 775,000	677,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date, first	Jul. 31, 2018
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date, Last	Mar. 31, 2021
National Electrical Benefit Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	530181657
Multiemployer plan number	001
Multiemployer plans, certified zone status	Green	Green
Multiemployer plans, certified zone status, date	Dec. 31, 2017	Dec. 31, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	No
Multiemployer plan, period contributions	$ 8,891,000	$ 6,366,000	5,271,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date, first	Oct. 31, 2017
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date, Last	Mar. 31, 2021
Sheet Metal Workers Pension Plan of Southern CA, AZ, and NV [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	956052257
Multiemployer plan number	001
Multiemployer plans, certified zone status	Yellow	Red
Multiemployer plans, certified zone status, date	Dec. 31, 2017	Dec. 31, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	$ 1,016,000	$ 1,087,000	714,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Jun. 30, 2018
Southwest Marine Pension Trust [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	956123404
Multiemployer plan number	001
Multiemployer plans, certified zone status	Red	Red
Multiemployer plans, certified zone status, date	Dec. 31, 2017	Dec. 31, 2016
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	$ 48,000	$ 50,000	26,000
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Jan. 31, 2019
Multiemployer Plan, Individually Insignificant Multiemployer Plans [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 22,066,000	19,835,000	$ 18,254,000
Operating Engineers Local 800 and WY Contractors Association Pension Plan for WY [Member] | Withdrawal from multiemployer defined benefit plan [Member]
Multiemployer Plans [Line Items]
Multiemployer discounted future quarterly payments		42,000
Multiemployer withdrawal liability prior accrual		16,400,000
Multiemployer plans, withdrawal obligation		$ 5,200,000
Minimum [Member]
Multiemployer Plans [Line Items]
Multiemployer plan yellow zone status funded percentage	65.00%
Multiemployer plan green zone status funded percentage	80.00%
Maximum [Member]
Multiemployer Plans [Line Items]
Multiemployer plan red zone status funded percentage	65.00%
Mulitiemployer plan yellow status funded percentage	80.00%