Short-Term Borrowings and Long-Term Debt Line of Credit (Details) - USD ($) $ in Thousands	12 Months Ended	15 Months Ended	33 Months Ended
	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2020	Mar. 30, 2017	Sep. 30, 2016	Mar. 31, 2016	Dec. 28, 2015	Sep. 30, 2015
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Dec. 31, 2020
Debt Instrument, Basis Spread on Variable Rate	1.25%
Line of Credit Facility, Maximum Borrowing Capacity	$ 500,000			$ 1,000,000			$ 500,000
Line of Credit Facility, Commitment Fee Percentage	0.175%
Line of Credit Facility, Current Borrowing Capacity	$ 500,000					$ 960,000
Line of Credit Facility, Additional Capacity Available	500,000						$ 250,000
Line of Credit Facility, Annual Principal Payment	500,000
Line of Credit Facility, Increase (Decrease), Net	460,000
Line of Credit Facility, Fair Value of Amount Outstanding						$ 40,000		$ 40,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
Line of Credit Facility [Line Items]
Long-term Debt, Gross	2,250,000
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] | 5.45% Junior Subordinated Notes [Member]
Line of Credit Facility [Line Items]
Long-term Debt, Gross	$ 500,000				$ 500,000
Subsequent Event [Member] | Debt to EBITDA [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Covenant Description		3.5 to 1	3.0 to 1
Minimum [Member] | EBITDA to Interest [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Covenant Description	4.0 to 1
Scenario, Actual [Member] | Debt to EBITDA [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Covenant Description	5.7 to 1
Scenario, Actual [Member] | EBITDA to Interest [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Covenant Description	2.6 to 1