Investment Real Estate - Investment Real Estate by Property-Type and Geographic Distribution (Detail) - Real Estate [Member]	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Property Type Concentration Risk [Member]
Concentration Risk [Line Items]
Investment real estate	100.00%	100.00%
Property Type Concentration Risk [Member] | Industrial [Member]
Concentration Risk [Line Items]
Investment real estate	11.20%	13.00%
Property Type Concentration Risk [Member] | Office [Member]
Concentration Risk [Line Items]
Investment real estate	33.40%	25.00%
Property Type Concentration Risk [Member] | Retail [Member]
Concentration Risk [Line Items]
Investment real estate	40.00%	44.10%
Property Type Concentration Risk [Member] | Other [Member]
Concentration Risk [Line Items]
Investment real estate	15.40%	17.90%
Geographic Distribution [Member]
Concentration Risk [Line Items]
Investment real estate	100.00%	100.00%
Geographic Distribution [Member] | East North Central [Member]
Concentration Risk [Line Items]
Investment real estate	12.30%	4.50%
Geographic Distribution [Member] | East South Central [Member]
Concentration Risk [Line Items]
Investment real estate	3.80%	4.60%
Geographic Distribution [Member] | Mountain [Member]
Concentration Risk [Line Items]
Investment real estate	8.00%	9.60%
Geographic Distribution [Member] | Pacific [Member]
Concentration Risk [Line Items]
Investment real estate	6.10%	7.10%
Geographic Distribution [Member] | South Atlantic [Member]
Concentration Risk [Line Items]
Investment real estate	10.90%	12.20%
Geographic Distribution [Member] | West South Central [Member]
Concentration Risk [Line Items]
Investment real estate	53.00%	55.60%
Geographic Distribution [Member] | Other [Member]
Concentration Risk [Line Items]
Investment real estate	5.90%	6.40%