HA Sustainable Infrastructure Capital, Inc. Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Summary of Fair Value and Carrying Value of Financial Assets and Liabilities (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Allowance for loan losses on financing receivables	$ 1.2	$ 1.2
Amortized costs	22.0	26.9
Financing Receivable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net fair value collateral impaired loan	0.8	0.8
Allowance for loan losses on financing receivables	$ 1.2	$ 1.2