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New Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Jan. 01, 2016
New Accounting Pronouncement Early Adoption [Line Items]
Accumulated earnings	$ 4,110,889		$ 4,110,889		$ 3,443,007
Cumulative adjustment to accumulated earnings	$ 5,092,208	$ 4,493,760	14,572,398	$ 12,735,501
Distributions from equity method investments reclassified from cash flows from investing activities to cash flows from operating activities			$ 262,670	155,937
5.00% Senior Notes [Member]
New Accounting Pronouncement Early Adoption [Line Items]
Debt Instrument Interest Rate Stated Percentage	5.00%		5.00%		5.00%
Accounting Standards Update 2016-01
New Accounting Pronouncement Early Adoption [Line Items]
Cumulative adjustment to accumulated earnings							$ 4,000
Accounting Standards Update 2016-15
New Accounting Pronouncement Early Adoption [Line Items]
Distributions from equity method investments reclassified from cash flows from investing activities to cash flows from operating activities				138,300
Cash paid for acquisitions reclassified from cash used in investing activities to cash used in financing activities				$ 23,300
Accounting Standards Update 2016-15 \| 5.00% Senior Notes [Member]
New Accounting Pronouncement Early Adoption [Line Items]
Deb instrument, redemption premium	$ 20,000		$ 20,000
Debt Instrument Interest Rate Stated Percentage	5.00%		5.00%
Adoption of new revenue recognition guidance [Member] \| ASU 2014-09 [Member]
New Accounting Pronouncement Early Adoption [Line Items]
Accumulated earnings					$ 94,622			$ 87,900
Cumulative adjustment to accumulated earnings					$ 5,600	$ 1,100