Significant Accounting Policies - Cost of Sales (Details) - USD ($) $ in Thousands	12 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Amortization related to intangible assets
Amortization of contract-based intangible assets	$ 114,797	$ 107,628	$ 101,751
Cost of sales - wholesale supply and marketing
Amortization related to intangible assets
Amortization of contract-based intangible assets	6,828	6,700	7,767
Cost of sales - wholesale supply and marketing \| Contract-based intangibles
Amortization related to intangible assets
Amortization of contract-based intangible assets	$ 6,800	$ 6,700	$ 7,800

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275

Name:	us-gaap_AmortizationOfIntangibleAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration