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Short-Term Borrowings and Long-Term Debt Schedule of Current Value of Long-term Debt (Details) - USD ($)	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Mar. 22, 2016	Jun. 26, 2014	Jan. 31, 2014
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount					$ 553,000
3.95% Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount					458,000.0000
4.75% Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount				$ 207,000.0000
5.625% Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Unamortized Premium					9,779,000
Debt Instrument, Unamortized Discount					6,260,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 2,250,000,000
Long-term Debt	2,221,867,000		$ 1,740,985,000
Debt Instrument, Fair Value Disclosure	(4,059,000)
Debt Instrument, Unamortized Discount (Premium), Net	(2,692,000)
Unamortized Debt Issuance Expense	34,884,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 2.7% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000,000
Long-term Debt	252,980,000		256,055,000
Debt Instrument, Fair Value Disclosure	(4,059,000)
Debt Instrument, Unamortized Discount	250,000
Unamortized Debt Issuance Expense	829,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 3.95% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000,000
Long-term Debt	248,265,000		248,028,000
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Discount	332,000
Unamortized Debt Issuance Expense	1,403,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 4.75% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	450,000,000
Long-term Debt	446,812,000		447,030,000
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Premium	0
Unamortized Debt Issuance Expense	3,188,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 5.45% Junior Subordinated Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	500,000,000	$ 500,000,000
Long-term Debt	483,895,000		0
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Premium	0
Unamortized Debt Issuance Expense	16,105,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 5.625% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	550,000,000				$ 150,000,000	$ 400,000,000
Long-term Debt	547,861,000		547,781,000
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Premium	(3,274,000)
Unamortized Debt Issuance Expense	5,413,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 6.375% Junior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000,000
Long-term Debt	242,054,000		$ 242,091,000
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Premium	0
Unamortized Debt Issuance Expense	$ 7,946,000