Financial Instruments and Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Debt (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	$ 9,427,124	$ 9,412,631
Fair Value of Debt	9,828,243	9,955,795
Credit Facilities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	11,658	317,392
Fair Value of Debt	11,660	317,496
Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	7,102,381	6,067,277
Fair Value of Debt	7,447,585	6,537,100
Term Loans and Unsecured Other Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	1,415,661	2,060,491
Fair Value of Debt	1,431,793	2,075,002
Secured Mortgages [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value of Debt	897,424	967,471
Fair Value of Debt	$ 937,205	$ 1,026,197