Multiemployer plans (Details 15) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Multiemployer Plans [Line Items]
Multiemployer plan red zone status funded percentage, maximum	65.00%
Multiemployer plan yellow zone status funded percentage, minimum	65.00%
Mulitiemployer plan yellow status funded percentage, maximum	80.00%
Multiemployer plan green zone status funded percentage, minimum	80.00%
Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 34,779		$ 29,158	$ 30,276
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, 2012		Dec. 31, 2011
Multiemployer plans, funding improvement plan and rehabilitation plan	No
Multiemployer plan, period contributions	5,171		2,700	1,933
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Dec. 31, 2014
IBEW Local 38 Pension Plan [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	346574238
Multiemployer plan number	001
Multiemployer plans, certified zone status	Yellow		Yellow
Multiemployer plans, certified zone status, date	Apr. 30, 2012		Apr. 30, 2011
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	2,771		1,469	1,277
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Apr. 27, 2014
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	Red		Red
Multiemployer plans, certified zone status, date	Jun. 30, 2012		Jun. 30, 2011
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	1,093		1,331	1,569
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Dec. 01, 2013
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. 29, 2012		Feb. 28, 2011
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	564		722	781
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Aug. 31, 2015
Laborers Pension Trust Fund for Northern California [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	946277608
Multiemployer plan number	001
Multiemployer plans, certified zone status	Yellow		Yellow
Multiemployer plans, certified zone status, date	May 31, 2012		May 31, 2011
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	567		628	413
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Jun. 30, 2012	[1]
Multiemployer plans, collective-bargaining, date range - initial	Jun. 30, 2011	[1]
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	Yellow		Yellow
Multiemployer plans, certified zone status, date	Apr. 30, 2012		Apr. 30, 2011
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	664		776	679
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Jun. 02, 2013
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, funding improvement plan and rehabilitation plan	No
Multiemployer plan, period contributions	5,603		4,841	4,826
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Dec. 31, 2016
Multiemployer plans, collective-bargaining, date range - initial	Aug. 31, 2011	[1]
OE Pension Trust Fund [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	946090764
Multiemployer plan number	001
Multiemployer plans, certified zone status	Yellow		Yellow
Multiemployer plans, certified zone status, date	Dec. 31, 2012		Dec. 31, 2011
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	1,156		1,367	1,035
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Mar. 31, 2016
Multiemployer plans, collective-bargaining, date range - initial	Jun. 30, 2010	[1]
Operating Engineers Local 800 and WY Contractors Association Pension Plan for WY [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	836011320
Multiemployer plan number	001
Multiemployer plans, certified zone status	Red		Red
Multiemployer plans, certified zone status, date	Dec. 31, 2012		Dec. 31, 2011
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	91		96	106
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Oct. 31, 2005	[1]
Operating Engineers Pension Trust [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Entity tax identification number	956032478
Multiemployer plan number	001
Multiemployer plans, certified zone status	Red		Red
Multiemployer plans, certified zone status, date	Jun. 30, 2012		Jun. 30, 2011
Multiemployer plans, funding improvement plan and rehabilitation plan	Implemented
Multiemployer plan, period contributions	761		458	343
Multiemployer plans, surcharge	No
Multiemployer plans, collective-bargaining arrangement, expiration date	Dec. 31, 2016
Multiemployer plans, collective-bargaining, date range - initial	Jun. 30, 2013
Multiemployer Plan, Individually Insignificant Multiemployer Plans [Member] | Multiemployer plans, pension [Member]
Multiemployer Plans [Line Items]
Multiemployer plan, period contributions	$ 16,338		$ 14,770	$ 17,314

[1] Plan includes collective bargaining agreements which have expired. The agreements contain provisions that automatically renew the existing contracts in lieu of a new negotiated collective bargaining agreement.