Segment and Geographical Information - Long Lived Assets and Continuing Operations by Country (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	$ 484,781	$ 427,300	$ 1,412,518	$ 1,246,161
Income (loss) before income taxes	111,435	100,200	308,160	271,432
Long-lived assets	30,592		30,592		$ 28,099
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	411,998	358,534	1,191,502	1,047,717
Income (loss) before income taxes	91,793	81,040	249,106	216,586
Long-lived assets	25,980		25,980		27,443
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	72,783	68,766	221,016	198,444
Income (loss) before income taxes	19,642	$ 19,160	59,054	$ 54,846
Long-lived assets	$ 4,612		$ 4,612		$ 656