Recent Accounting Pronouncements (Details) - 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
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred gain on sale of real estate					$ 712
Unrealized gain on investment in equity securities	$ 0	$ 3,279	$ 0	$ 14,389
Equity in earnings of an investee	94	31	(1,112)	533
Increase in cash provided by operating activities			120,165	95,914
Restricted cash	$ 2,354	$ 509	$ 2,354	509	3,111
ASU 2016-01 | Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized gain on investment in equity securities					45,116
ASU 2016-15
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accretion in equity of earnings				2,209
ASU 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in cash provided by operating activities				$ 21
SIR and AIC | ASU 2016-01 | Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity in earnings of an investee					$ 15,165