Interest and Other Investment Income, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest and Other Income [Abstract]
Dividends on marketable securities	$ 13,276	$ 13,135	$ 12,836
Mark-to-market of investments in our deferred compensation plan	6,932	5,213	111
Interest on loans receivable	4,352	3,890	6,371
Other, net	13,233	7,310	7,922
Interest and other investment (loss) income, net	$ 37,793	$ 29,548	$ 27,240