Income Taxes - Components of Net Income Before Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[2]	Sep. 30, 2016	[2]	Jun. 30, 2016	[2]	Mar. 31, 2016	[2]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of net income before income taxes
Domestic																	$ 89,095	$ 98,966	$ 93,390
Foreign																	(1,385)	1,033	2,951
Income before income taxes	$ 21,671		$ 23,386		$ 20,303		$ 22,350		$ 19,461		$ 30,159		$ 17,285		$ 33,094		$ 87,710	$ 99,999	$ 96,341

[1] During the first, second, third, and fourth quarters of 2017 we incurred restructuring and reorganization charges of $0.2 million, $2.7 million, $1.6 million, and $4.2 million, respectively, or $0.01, $0.05, $0.03, and $0.09 per diluted share (see Note 6).

[2] During the first, second, and fourth quarters of 2016 we incurred restructuring and reorganization charges of $(5.7) million, $5.3 million, and $1.0 million, respectively, or $(0.11), $0.10, and $0.02 per diluted share. The negative restructuring and reorganization charges in the first quarter reflect the gain on disposition of business operations (see Note 6).