Short-Term Borrowings and Long-Term Debt Schedule of Current Value of Long-term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 250,636	$ 0
Long-term debt, net	1,971,131	2,221,810
Long-term Debt, Fair Value	2,143,373
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	2,250,000
Long-term Debt	2,221,767	2,221,810
Debt Instrument, Fair Value Disclosure	(902)
Debt Instrument, Unamortized Discount (Premium), Net	(2,749)
Unamortized Debt Issuance Expense	31,884
Long-term Debt, Maturities, Repayments of Principal in Year Three	250,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	2,000,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 2.7% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000
Long-term Debt	250,636	251,641
Debt Instrument, Fair Value Disclosure	(902)
Debt Instrument, Unamortized Discount	59
Unamortized Debt Issuance Expense	207
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 3.95% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000
Long-term Debt	248,680	248,502
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Discount	252
Unamortized Debt Issuance Expense	1,068
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 4.75% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	450,000
Long-term Debt	447,432	447,166
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Premium	0
Unamortized Debt Issuance Expense	2,568
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
Long-term Debt	484,608	484,303
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Premium	0
Unamortized Debt Issuance Expense	15,392
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 5.625% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	550,000
Long-term Debt	548,000	547,940
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Premium	(3,060)
Unamortized Debt Issuance Expense	5,060
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] \| 6.375% Junior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000
Long-term Debt	242,411	$ 242,258
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Premium	0
Unamortized Debt Issuance Expense	7,589
Current Portion of Long-Term Debt \| Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000
Debt Instrument, Fair Value Disclosure	(902)
Debt Instrument, Unamortized Discount	59
Unamortized Debt Issuance Expense	207
Long-term Debt \| Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	2,000,000
Debt Instrument, Fair Value Disclosure	0
Debt Instrument, Unamortized Discount (Premium), Net	(2,808)
Unamortized Debt Issuance Expense	$ 31,677

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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