Segment and Geographical Information - Long Lived Assets and Continuing Operations by Country (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	$ 525,815	$ 504,902	$ 1,050,751	$ 999,890
Income (loss) before income taxes	134,062	127,460	228,607	229,828
Long-lived assets		48,334		48,334	$ 46,116
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	452,864	430,093	895,999	850,086
Income (loss) before income taxes	109,153	106,723	182,099	187,457
Long-lived assets		43,871		43,871	41,200
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	72,951	74,809	154,752	149,804
Income (loss) before income taxes	$ 24,909	20,737	$ 46,508	42,371
Long-lived assets		$ 4,463		$ 4,463	$ 4,916