Components of Utility Plant by Category (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Public Utility, Property, Plant and Equipment [Line Items]
Construction work in progress	$ 349,496	$ 460,529
Utility plant, gross	15,242,165	14,232,047
Less accumulated depreciation	3,657,221	3,360,005
Utility plant, net	11,584,944	10,872,042
Water Plant | Land And Other Non-Depreciable Assets
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant excluding Construction work in progress	129,953	128,286
Water Plant | Sources Of Supply
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant excluding Construction work in progress	623,015	565,301
Water Plant | Sources Of Supply | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	16 years
Water Plant | Sources Of Supply | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	147 years
Water Plant | Treatment And Pumping Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant excluding Construction work in progress	2,944,178	2,665,525
Water Plant | Treatment And Pumping Facilities | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	3 years
Water Plant | Treatment And Pumping Facilities | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	101 years
Water Plant | Transmission And Distribution Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant excluding Construction work in progress	7,033,958	6,632,828
Water Plant | Transmission And Distribution Facilities | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	5 years
Water Plant | Transmission And Distribution Facilities | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	120 years
Water Plant | Services, Meters And Fire Hydrants
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant excluding Construction work in progress	2,729,679	2,539,696
Water Plant | Services, Meters And Fire Hydrants | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	9 years
Water Plant | Services, Meters And Fire Hydrants | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	93 years
Water Plant | General Structures And Equipment
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant excluding Construction work in progress	865,992	693,880
Water Plant | General Structures And Equipment | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	3 years
Water Plant | General Structures And Equipment | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	112 years
Wastewater Plant
Public Utility, Property, Plant and Equipment [Line Items]
Utility plant excluding Construction work in progress	$ 565,894	$ 546,002
Wastewater Plant | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	2 years
Wastewater Plant | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Range of Remaining Useful Lives	115 years