Real Estate and Other Investments - Investment Real Estate by Property-Type and Geographic Distribution (Detail) - Real Estate	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Property Type Concentration Risk
Concentration Risk [Line Items]
Percent of fair value	100.00%	100.00%
Property Type Concentration Risk | Industrial
Concentration Risk [Line Items]
Percent of fair value	12.80%	13.10%
Property Type Concentration Risk | Office
Concentration Risk [Line Items]
Percent of fair value	37.80%	37.30%
Property Type Concentration Risk | Retail
Concentration Risk [Line Items]
Percent of fair value	37.00%	37.00%
Property Type Concentration Risk | Other
Concentration Risk [Line Items]
Percent of fair value	12.40%	12.60%
Geographic Distribution
Concentration Risk [Line Items]
Percent of fair value	100.00%	100.00%
Geographic Distribution | East North Central
Concentration Risk [Line Items]
Percent of fair value	5.60%	5.60%
Geographic Distribution | East South Central
Concentration Risk [Line Items]
Percent of fair value	5.70%	5.40%
Geographic Distribution | Mountain
Concentration Risk [Line Items]
Percent of fair value	11.80%	11.90%
Geographic Distribution | Pacific
Concentration Risk [Line Items]
Percent of fair value	7.10%	7.30%
Geographic Distribution | South Atlantic
Concentration Risk [Line Items]
Percent of fair value	15.00%	13.80%
Geographic Distribution | West South Central
Concentration Risk [Line Items]
Percent of fair value	53.00%	53.80%
Geographic Distribution | Other
Concentration Risk [Line Items]
Percent of fair value	1.80%	2.20%