Segment and Geographical Information - Long Lived Assets and Continuing Operations by Country (Details) - 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
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	$ 442,942	$ 427,300	$ 413,696	$ 405,164	$ 393,864	$ 383,626	$ 378,994	$ 362,600	$ 1,689,102	$ 1,519,084	$ 1,404,199
Income (loss) before income taxes	108,089	$ 100,200	$ 96,389	$ 74,842	87,064	$ 88,437	$ 91,880	$ 70,212	379,520	337,595	290,981
Long-lived assets	28,099				27,465				28,099	27,465	29,408
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues									1,419,658	1,281,580	1,172,508
Income (loss) before income taxes									299,764	269,791	225,920
Long-lived assets	27,443				26,685				27,443	26,685	28,621
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues									269,444	237,504	231,691
Income (loss) before income taxes									79,756	67,804	65,061
Long-lived assets	$ 656				$ 780				$ 656	$ 780	$ 787