Financial Instruments and Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	$ 9,412,631	$ 10,608,294
Fair Value of Debt	9,955,795	11,219,716
Credit Facilities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	317,392	35,023
Fair Value of Debt	317,496	35,061
Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	6,067,277	6,417,492
Fair Value of Debt	6,537,100	6,935,485
Term Loans and Unsecured Other Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	2,060,491	1,499,001
Fair Value of Debt	2,075,002	1,510,661
Secured Mortgages [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	808,096	979,585
Fair Value of Debt	867,067	1,055,020
Secured Mortgage Debt of Consolidated Entities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	159,375	1,677,193
Fair Value of Debt	$ 159,130	$ 1,683,489