DEBT (Details) - USD ($)			1 Months Ended	6 Months Ended
	May 02, 2016	May 09, 2011	May 31, 2015	Sep. 30, 2017	Mar. 31, 2017
Debt Disclosure [Line Items]
Initiation date of credit agreement		May 09, 2011
Amendment date of Credit Agreement	Apr. 28, 2017
Credit facility expiration date				May 09, 2020
Commitment fee percentage				0.37%
Outstanding borrowings under line of credit				$ 0	$ 0
Minimum required liquid funds				$ 300,000,000.0
Line Of Credit Facility Interest Rate Spread Below The Banks Announced Prime Rate [Member]
Debt Disclosure [Line Items]
Spread for interest rate				1.20%
Line Of Credit Facility Interest Rate Spread Above A Daily One Month LIBOR Rate [Member]
Debt Disclosure [Line Items]
Spread for interest rate				1.40%
Senior Notes [Member] | 5.50% Senior Notes [Member]
Debt Disclosure [Line Items]
Principal amount of debt issued			$ 500,000,000.0
Stated interest rate of debt instrument			5.50%	5.50%
Proceeds from issuance of senior notes, net of issuance costs			$ 488,400,000
Debt issuance costs			$ 11,600,000
Debt redemption percentage price, Specified Price			105.50%
Repurchase price, percentage of principal amount			101.00%
Minimum [Member] | Senior Notes [Member] | 5.50% Senior Notes [Member]
Debt Disclosure [Line Items]
Debt redemption notice period			30 days
Maximum [Member] | Senior Notes [Member] | 5.50% Senior Notes [Member]
Debt Disclosure [Line Items]
Debt redemption notice period			60 days
Percentage of debt redeemed			35.00%
Level 2 [Member] | Fair Value [Member] | Senior Notes [Member] | 5.50% Senior Notes [Member]
Debt Disclosure [Line Items]
Long-term debt, 5.50% Senior Notes				$ 520,205,000	505,150,000
Level 2 [Member] | Carrying Value [Member] | Senior Notes [Member] | 5.50% Senior Notes [Member]
Debt Disclosure [Line Items]
Long-term debt, 5.50% Senior Notes				$ 491,784,000	$ 491,059,000
Revolving Credit Facility [Member]
Debt Disclosure [Line Items]
Maximum borrowing capacity of unsecured revolving credit facility	$ 100,000,000.0