Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Land	$ 33,814	$ 26,339
Buildings	473,489	429,039
Leasehold improvements	2,816,675	2,495,070
Equipment and information systems, including internally developed software	2,352,246	2,182,912
New center and capital asset projects in progress	576,651	429,037
Property, plant and equipment, gross, total	6,252,875	5,562,397
Less accumulated depreciation	(3,103,662)	(2,698,276)
Property and equipment, net	$ 3,149,213	$ 2,864,121