Segment Information - Revenue by Geographic Area (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 301,207	$ 278,828
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	144,838	137,645
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	11,654	10,582
Total Americas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	156,492	148,227
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	47,025	42,610
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	60,302	53,439
Europe, the Middle East and Africa ("EMEA") [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	107,327	96,049
Japan [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	12,826	12,640
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	24,562	21,912
Total Asia & Australia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 37,388	$ 34,552