Goodwill and Intangible Assets - Goodwill (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2017 USD ($)
Goodwill
Balance at beginning of the period	$ 740,886
Goodwill additions	24,522
Foreign exchange impact	(1,953)
Balance at end of the period	763,455
GSG
Goodwill
Balance at beginning of the period	361,761
Goodwill additions	24,522
Foreign exchange impact	(545)
Balance at end of the period	385,738
CIG
Goodwill
Balance at beginning of the period	379,125
Goodwill additions	0
Foreign exchange impact	(1,408)
Balance at end of the period	$ 377,717