Acquisitions - Summary of Components of Purchase Consideration Transferred and Preliminary Allocation of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Goodwill	$ 474,990	$ 477,532	$ 412,846
All Acquisitions Other Than Berkeley Point Acquisition [Member]
Assets
Cash and cash equivalents		3,903
Goodwill		64,291
Intangibles assets, net		3,188
Other assets		9,234
Total Assets		80,616
Current liabilities
Accounts payable, accrued expenses and other liabilities		7,119
Total Liabilities		7,119
Noncontrolling interest		19,145
Net assets acquired		$ 54,352