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Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements
Increase to cash flows provided by operating activities	$ (36,617)	$ 99,921
Decrease to cash flows provided by investing activities	428,003	(1,038,643)
Decrease to net cash provided by investing activities	(579,215)	(440,928)
Reconciliation of Cash, Cash Equivalents, and Restricted Cash
Cash and cash equivalents	757,384	1,912,448		$ 657,688	$ 328,744
Restricted cash included in deferred expenses and other assets, net	34,974	21,690		20,045	25,883
Total cash, cash equivalents and restricted cash reported in the consolidated statements of cash flows	792,358	1,934,138		677,733	354,627
Impact of Adoption on Consolidated Financial Statements
Other investments	302,318			321,241
Total assets	5,079,989			4,731,078
Retained earnings (deficit)	(2,350,438)			(2,470,564)
Total equity	$ 1,192,202	923,889		$ 914,249	$ 1,059,684
Accounting Standards Update 2016-15
New Accounting Pronouncements
Increase to cash flows provided by operating activities		11,800
Decrease to cash flows provided by investing activities		11,800
Accounting Standards Update 2016-18
New Accounting Pronouncements
Increase to cash flows provided by operating activities		1,300
Decrease to net cash provided by investing activities		$ 5,500
Accounting Standards Update 2017-05
Impact of Adoption on Consolidated Financial Statements
Other investments			$ 397,110
Total assets			4,806,947
Retained earnings (deficit)			(2,394,695)
Total equity			990,118
Accounting Standards Update 2017-05 \| Impact of Adoption of ASU
Impact of Adoption on Consolidated Financial Statements
Other investments			75,869
Total assets			75,869
Retained earnings (deficit)			75,869
Total equity			$ 75,869