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Commitments, Contingencies and Off-Balance Sheet Arrangements - Litigation, Regulatory and Taxation Matters- Additional Information (Detail)
$ in Millions
 6 Months Ended 24 Months Ended
Jun. 30, 2018
Companies
Dec. 31, 2017
USD ($)
Minimum [Member]    
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]    
Number of micro-captive insurance companies organized or managed | Companies 100  
Micro-captive operations, net earnings and EBITDAC, description In the period from 2015 to 2017, our micro-captive operations contributed less than $3.5 million of net earnings and less than $5.0 million in EBITDAC to our consolidated results in any one year.  
Maximum [Member]    
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]    
Micro-captive operations, net earnings   $ 3.5
Micro-captive operations, EBITDAC   $ 5.0