Variable Interest Entities - Summary of VIEs (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 MW
Variable Interest Entity, Not Primary Beneficiary [Member] | Pilesgrove [Member] | New Jersey [Member]
Variable Interest Entity [Line Items]
Generating Capacity Owned	9
Year of Initial Investment	2010
Maximum Exposure to Loss (In Millions)	$ 26,000,000
Variable Interest Entity, Not Primary Beneficiary [Member] | Mesquite Solar 1 [Member] | Arizona [Member]
Variable Interest Entity [Line Items]
Generating Capacity Owned	83
Power Purchase Agreement Term in Years	20 years
Year of Initial Investment	2013
Maximum Exposure to Loss (In Millions)	111,000,000
Variable Interest Entity, Not Primary Beneficiary [Member] | Copper Mountain Solar 2 [Member] | Nevada [Member]
Variable Interest Entity [Line Items]
Generating Capacity Owned	75
Power Purchase Agreement Term in Years	25 years
Year of Initial Investment	2013
Maximum Exposure to Loss (In Millions)	80,000,000
Variable Interest Entity, Not Primary Beneficiary [Member] | Copper Mountain Solar 3 [Member] | Nevada [Member]
Variable Interest Entity [Line Items]
Generating Capacity Owned	128
Power Purchase Agreement Term in Years	20 years
Year of Initial Investment	2014
Maximum Exposure to Loss (In Millions)	175,000,000
Variable Interest Entity, Not Primary Beneficiary [Member] | California Solar [Member] | California [Member]
Variable Interest Entity [Line Items]
Generating Capacity Owned	55
Power Purchase Agreement Term in Years	25 years
Year of Initial Investment	2012
Maximum Exposure to Loss (In Millions)	81,000,000
Variable Interest Entity, Not Primary Beneficiary [Member] | Broken Bow II [Member] | Nebraska [Member]
Variable Interest Entity [Line Items]
Generating Capacity Owned	37
Power Purchase Agreement Term in Years	25 years
Year of Initial Investment	2014
Maximum Exposure to Loss (In Millions)	57,000,000
Variable Interest Entity, Primary Beneficiary [Member] | Texas Solar 4 [Member] | Texas [Member]
Variable Interest Entity [Line Items]
Generating Capacity Owned	32
Power Purchase Agreement Term in Years	25 years
Year of Initial Investment	2014
Maximum Exposure to Loss (In Millions)	$ 58,000,000