Segment Reporting - Revenues Based on Location of Legal Entity (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 467,711	$ 448,962	$ 444,715	$ 446,344	$ 441,920	$ 438,042	$ 460,147	$ 470,285	$ 1,807,732	$ 1,810,394	$ 1,779,149
United States
Segment Reporting Information [Line Items]
Revenues									1,262,682	1,298,492	1,281,444
United Kingdom
Segment Reporting Information [Line Items]
Revenues									251,843	246,236	236,925
All other foreign countries
Segment Reporting Information [Line Items]
Revenues									$ 293,207	$ 265,666	$ 260,780