Shareholder Report, Holdings (Details)	Jan. 31, 2025
C000118601 [Member] | Prologis Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	9.10%	[1]
C000118601 [Member] | Equinix Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	7.20%	[1]
C000118601 [Member] | American Tower Corp [Member]
Holdings [Line Items]
Percent of Net Asset Value	6.30%	[1]
C000118601 [Member] | AvalonBay Communities Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	5.30%	[1]
C000118601 [Member] | Welltower Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.90%	[1]
C000118601 [Member] | Simon Property Group Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.60%	[1]
C000118601 [Member] | Mid America Apartment Communities Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.10%	[1]
C000118601 [Member] | SBA Communications Corp [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.10%	[1]
C000118601 [Member] | VICI Properties Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	3.70%	[1]
C000118601 [Member] | Extra Space Storage Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	3.60%	[1]
C000118601 [Member] | Specialized REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	37.70%	[2]
C000118601 [Member] | Retail REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	14.60%	[2]
C000118601 [Member] | Residential REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	13.70%	[2]
C000118601 [Member] | Health Care REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	12.60%	[2]
C000118601 [Member] | Industrial REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	12.50%	[2]
C000118601 [Member] | Office REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	3.40%	[2]
C000118601 [Member] | Hotel And Resort REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	2.50%	[2]
C000118601 [Member] | Diversified REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	1.40%	[2]
C000118601 [Member] | Short-Term Investments [Member]
Holdings [Line Items]
Percent of Net Asset Value	1.20%	[2]
C000118601 [Member] | Other Assets Less Liabilities [Member]
Holdings [Line Items]
Percent of Net Asset Value	0.40%	[2]
C000118599 [Member] | Prologis Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	9.10%	[3]
C000118599 [Member] | Equinix Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	7.20%	[3]
C000118599 [Member] | American Tower Corp [Member]
Holdings [Line Items]
Percent of Net Asset Value	6.30%	[3]
C000118599 [Member] | AvalonBay Communities Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	5.30%	[3]
C000118599 [Member] | Welltower Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.90%	[3]
C000118599 [Member] | Simon Property Group Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.60%	[3]
C000118599 [Member] | Mid America Apartment Communities Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.10%	[3]
C000118599 [Member] | SBA Communications Corp [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.10%	[3]
C000118599 [Member] | VICI Properties Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	3.70%	[3]
C000118599 [Member] | Extra Space Storage Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	3.60%	[3]
C000118599 [Member] | Specialized REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	37.70%	[4]
C000118599 [Member] | Retail REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	14.60%	[4]
C000118599 [Member] | Residential REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	13.70%	[4]
C000118599 [Member] | Health Care REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	12.60%	[4]
C000118599 [Member] | Industrial REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	12.50%	[4]
C000118599 [Member] | Office REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	3.40%	[4]
C000118599 [Member] | Hotel And Resort REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	2.50%	[4]
C000118599 [Member] | Diversified REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	1.40%	[4]
C000118599 [Member] | Short-Term Investments [Member]
Holdings [Line Items]
Percent of Net Asset Value	1.20%	[4]
C000118599 [Member] | Other Assets Less Liabilities [Member]
Holdings [Line Items]
Percent of Net Asset Value	0.40%	[4]
C000118600 [Member] | Prologis Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	9.10%	[5]
C000118600 [Member] | Equinix Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	7.20%	[5]
C000118600 [Member] | American Tower Corp [Member]
Holdings [Line Items]
Percent of Net Asset Value	6.30%	[5]
C000118600 [Member] | AvalonBay Communities Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	5.30%	[5]
C000118600 [Member] | Welltower Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.90%	[5]
C000118600 [Member] | Simon Property Group Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.60%	[5]
C000118600 [Member] | Mid America Apartment Communities Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.10%	[5]
C000118600 [Member] | SBA Communications Corp [Member]
Holdings [Line Items]
Percent of Net Asset Value	4.10%	[5]
C000118600 [Member] | VICI Properties Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	3.70%	[5]
C000118600 [Member] | Extra Space Storage Inc [Member]
Holdings [Line Items]
Percent of Net Asset Value	3.60%	[5]
C000118600 [Member] | Specialized REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	37.70%	[6]
C000118600 [Member] | Retail REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	14.60%	[6]
C000118600 [Member] | Residential REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	13.70%	[6]
C000118600 [Member] | Health Care REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	12.60%	[6]
C000118600 [Member] | Industrial REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	12.50%	[6]
C000118600 [Member] | Office REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	3.40%	[6]
C000118600 [Member] | Hotel And Resort REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	2.50%	[6]
C000118600 [Member] | Diversified REITs [Member]
Holdings [Line Items]
Percent of Net Asset Value	1.40%	[6]
C000118600 [Member] | Short-Term Investments [Member]
Holdings [Line Items]
Percent of Net Asset Value	1.20%	[6]
C000118600 [Member] | Other Assets Less Liabilities [Member]
Holdings [Line Items]
Percent of Net Asset Value	0.40%	[6]

[1] Excludes short-term securities.

[2] For purposes of this report, industry sub-classifications may differ from those utilized by the Fund for compliance purposes.

[3] Excludes short-term securities.

[4] For purposes of this report, industry sub-classifications may differ from those utilized by the Fund for compliance purposes.

[5] Excludes short-term securities.

[6] For purposes of this report, industry sub-classifications may differ from those utilized by the Fund for compliance purposes.