Goodwill and Acquired Intangible Assets (Goodwill and Acquired Intangible Assets) (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Jul. 02, 2016	Sep. 30, 2015
Goodwill (not amortized), Net Book Value		$ 1,182,772	$ 1,169,813	$ 1,169,660	$ 1,069,041
Intangible assets with finite lives (amortized), Gross Carrying Amount		768,932	756,132
Intangible assets with finite lives (amortized), Accumulated Amortization		511,024	445,827
Intangible assets with finite lives (amortized), Net Book Value		257,908	310,305
Total goodwill and acquired intangible assets		1,440,680	1,480,118
Purchased Software [Member]
Intangible assets with finite lives (amortized), Gross Carrying Amount	[1]	362,955	354,595
Intangible assets with finite lives (amortized), Accumulated Amortization	[1]	228,377	199,192
Intangible assets with finite lives (amortized), Net Book Value	[1]	$ 134,578	155,403
Weighted average useful lives (in years)		9 years
Capitalized Software [Member]
Intangible assets with finite lives (amortized), Gross Carrying Amount	[1]	$ 22,877	22,877
Intangible assets with finite lives (amortized), Accumulated Amortization	[1]	22,877	22,877
Intangible assets with finite lives (amortized), Net Book Value	[1]	0	0
Customer Lists [Member]
Intangible assets with finite lives (amortized), Gross Carrying Amount	[1]	359,932	355,698
Intangible assets with finite lives (amortized), Accumulated Amortization	[1]	241,554	206,515
Intangible assets with finite lives (amortized), Net Book Value	[1]	$ 118,378	149,183
Weighted average useful lives (in years)		10 years
Trademarks And Trade Names [Member]
Intangible assets with finite lives (amortized), Gross Carrying Amount	[1]	$ 19,138	19,007
Intangible assets with finite lives (amortized), Accumulated Amortization	[1]	14,186	13,323
Intangible assets with finite lives (amortized), Net Book Value	[1]	$ 4,952	5,684
Weighted average useful lives (in years)		10 years
Other [Member]
Intangible assets with finite lives (amortized), Gross Carrying Amount	[1]	$ 4,030	3,955
Intangible assets with finite lives (amortized), Accumulated Amortization	[1]	4,030	3,920
Intangible assets with finite lives (amortized), Net Book Value	[1]	$ 0	$ 35

[1] The weighted average useful lives of purchased software, customer lists and relationships, and trademarks and trade names with a remaining net book value are 9 years, 10 years, and 10 years, respectively.