Segment Information - Revenue by Geographic Area (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 316,089	$ 290,596	$ 617,296	$ 569,424
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	155,997	134,719	300,835	272,364
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	11,298	11,388	22,952	21,970
Total Americas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	167,295	146,107	323,787	294,334
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	47,237	43,976	94,262	86,586
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	63,786	63,435	124,088	116,874
Europe, the Middle East and Africa ("EMEA") [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	111,023	107,411	218,350	203,460
Japan [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	13,237	13,563	26,063	26,203
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	24,534	23,515	49,096	45,427
Total Asia & Australia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 37,771	$ 37,078	$ 75,159	$ 71,630