Real Estate and Other Investments - Schedule of Investment in and Equity in Earnings of Unconsolidated Affiliates (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Total investments in unconsolidated affiliates	$ 571,897				$ 484,207				$ 571,897	$ 484,207
Income from operations									7,595	16,663	$ 19,005
Net gain on sales									13,686	70,011	38,195
Equity in earnings of unconsolidated affiliates	2,376	$ 13,029	$ 6,421	$ (545)	42,474	$ 22,387	$ 12,313	$ 9,500	21,281	86,674	$ 57,200
Real Estate
Schedule of Equity Method Investments [Line Items]
Total investments in unconsolidated affiliates	386,981				359,130				386,981	359,130
Equity and Fixed Income
Schedule of Equity Method Investments [Line Items]
Total investments in unconsolidated affiliates	156,121				95,819				156,121	95,819
Other
Schedule of Equity Method Investments [Line Items]
Total investments in unconsolidated affiliates	$ 28,795				$ 29,258				$ 28,795	$ 29,258