Acquisitions and Dispositions - KASW - Additional Information (Details) - USD ($)		3 Months Ended								11 Months Ended	12 Months Ended
	Jan. 29, 2015	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, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Revenue included in consolidated statements of operations and comprehensive income		$ 653,664,000	$ 611,870,000	$ 626,115,000	$ 540,317,000	$ 309,879,000	$ 275,659,000	$ 261,994,000	$ 255,658,000		$ 2,431,966,000	$ 1,103,190,000	$ 896,377,000
Operating income (loss)		$ 140,837,000	$ 129,072,000	$ 138,685,000	$ 110,151,000	$ 92,475,000	$ 72,897,000	$ 64,007,000	$ 57,929,000		$ 518,745,000	$ 287,308,000	206,107,000
KASW [Member]
Business Acquisition [Line Items]
Purchase price of entities acquired	$ 70,800,000
Acquisition related costs													$ 0
Acquisition date	Jan. 29, 2015
Revenue included in consolidated statements of operations and comprehensive income										$ 19,600,000
Operating income (loss)										$ 9,300,000