Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Reporting business segments | Segment	2
Reserve for uncollectible accounts	$ 8.5	$ 5.4
State tax net operating losses, valuation allowance	0.1
Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Impact on consolidated financial statements due to adoption of topic 606	3.0	$ 3.0
State [Member]
Summary Of Significant Accounting Policies [Line Items]
Incentive tax credit carryforwards	$ 1.6
Leasehold Improvements [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated life, description	The shorter of useful life or lease term.
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Ownership percentage	50.00%
Maximum [Member] | Customer Concentration Risk [Member] | Revenue [Member]
Summary Of Significant Accounting Policies [Line Items]
Minimum percentage of revenue accounted for by one customer	10.00%	10.00%	10.00%
Maximum [Member] | Building And Improvement [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated life	40 years
Maximum [Member] | Computer Equipment and Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated life	10 years
Maximum [Member] | Furniture and Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated life	10 years
Maximum [Member] | Transportation Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated life	15 years
Maximum [Member] | Software Development [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangible assets estimated useful life	10 years