Acquisitions (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Other Payments to Acquire Businesses	$ 901,000
Payments to Acquire Businesses, Gross	37,112,000	$ 37,112,000
Business Acquisition, Contingent Consideration, at Fair Value	$ 20,617,000	20,617,000	$ 10,000,000	$ 0
Business Combination, Assets and Liabilities Arising from Contingencies, Amount Recognized, Net		$ 10,617,000