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Segment Information - Summary of Financial Information by Operating Segment, Assets (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Assets:    
Assets $ 6,785,517 $ 6,172,504
U.S. Facilities [Member] | Operating Segment [Member]    
Assets:    
Assets 3,950,684 3,779,040
U.K. Facilities [Member] | Operating Segment [Member]    
Assets:    
Assets 2,633,347 2,175,809
Corporate and Other [Member] | Corporate Reconciling Items and Eliminations [Member]    
Assets:    
Assets $ 201,486 $ 217,655