Segment and Geographical Information - Long Lived Assets and Operations by Country (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	$ 413,696	$ 378,994	$ 818,860	$ 741,593
Income (loss) before income taxes	96,389	91,880	171,230	162,092
Long-lived assets	31,174		31,174		$ 27,465
United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	349,388	319,154	689,182	624,056
Income (loss) before income taxes	77,511	73,653	135,543	128,610
Long-lived assets	30,402		30,402		26,685
Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenues	64,308	59,840	129,678	117,537
Income (loss) before income taxes	18,878	$ 18,227	35,687	$ 33,482
Long-lived assets	$ 772		$ 772		$ 780